Slike strani
PDF
ePub

for a purpose; while for the benefit of those who desire to know more of the inter-relationship of the fungi enumerated, a classified list is given of all the species, arranged under their respective families, including the distribution and name of the host.

For the general reader, who is not specially interested in either insects or fungi, there is a considerable amount of interesting information bearing on such subjects as vegetable caterpillars, vegetable wasps, foul-brood of bees, &c., and the interest is not lessened by following the transition from the romantic and highly imaginative accounts given by early travellers of these productions, to the statements in accordance with modern knowledge. There is a slip on p. 35; Cordyceps Sheeringii should be C. Sherringii. The indices are very complete and the figures, excepting one on p. 10, good.

Notes on Qualitative Chemical Analysis. By P. Lakshmi Narasu Nayudu, B.A. (Madras: K. Murugesa Chetty, 1892.)

It is interesting to meet with books such as this, which serve to indicate how the study of chemistry is progressing in the colonies and dependencies of the empire. The author sets out with the endeavour to keep the rationale of the various processes of qualitative analysis well to the front, as in this way he considers the value of the study as a means of scientific training can alone be brought out. Group-reagents and the reasons for their use are first discussed as a preliminary to a somewhat exhaustive study of the reactions of the different basic and acid radicles. At the end of each group tables are given showing at a glance the behaviour of the radicles towards the various reagents.

In

It is somewhat astonishing that after such a minute study of the reactions of all the more common radicles, the author should give no schemes for the separation of the constituents of the different group-precipitates. spite of the fact that under each radicle he gives as many, if not more, reactions than are given in the larger works on qualitative analysis, he contents himself with merely going through the examination of a simple salt. The expenditure of but little space would remedy this omission, which limits the sphere of usefulness of the book. It is to be noted also that film-tests find no place in the system adopted.

It may be said that the author adheres well to his purpose of showing why any particular operation is performed. The book contains a large amount of useful information. Occasionally, however, the mode in which it is stated is peculiar. "In the cold” is an expression commonly used in speaking of a reaction. The use of "in the heat," a term often employed by the author, is, on the other hand, uncommon. To speak, too, of "neutral solutions of zinc salts containing strong acids" is confusing. In some cases, as when using bodies like potassium metantimoniate or sodium hydrogen tartrate, it would be advisable to give the name as well as the formula: it isn't every student who is acquainted with such substances. It is erroneous to say that fluorine does not combine with carbon even at a high temperature. According to Moissan, all the allotropes of carbon, except the diamond, unite with fluorine, indeed some of the forms are, in the cold, spontaneously inflammable in the gas.

66

66

mate

The following typographical errors are omitted in the list of errata. On p. 47 "meterially" should be rially," "gSo," &c. should be " MgSo," &c. on p. 58, and Ba P2O" is given for "Ba,P,O," on p. 69. Science Instruments. Catalogue of Scientific Apparatus and Reagents manufactured and sold by Brady and Martin. (Newcastle-on-Tyne, 1892.)

AT the present time, when almost all branches of experimental science are growing so rapidly, and new and improved pieces of apparatus are continually coming

into existence, it is satisfactory to find that instrument makers are trying to keep pace with the times, and to afford purchasers the means of ascertaining with the minimum trouble what apparatus can be obtained to serve a par ticular end. This catalogue is an instance that such is the case. It is a well-bound book, profusely and clearly illustrated. The different kinds of apparatus, useful both for teaching and for technical purposes, are well classified To prevent mistakes in ordering, each piece of apparatus is separately numbered, and where a new form is figured. a few lines are added explanatory of the principle involved.

The instruments quoted belong to various branches of experimental science-chemistry, bacteriology, physics, mechanics, and meteorology. A selection of instruments made by the Cambride company, and miscel laneous apparatus, diagrams, chemical reagents, &c., are also included.

The sections on bacteriology and gas analysis are especially full, and indicate the interest at present taken in these departments.

A table of contents and an index are supplied. On p. 145 "Irish" is misprinted for "Iris"; and what is termed an optical bank, on p. 164, is usually called as "optical bench."

LETTERS TO THE EDITOR.

The Editor does not hold himself responsible for opinions ex pressed by his correspondents. Neither can he undertak to return, or to correspond with the writers of, rejected manuscripts intended for this or any other part of NATURL No notice is taken of anonymous communications.]

Universities and Research.

AT the discussion in Edinburgh on the proposed National Laboratory, Lord Kelvin and Sir Geo. Stokes took marked exception to my contention that the primary business of Un versities was research, contending that it was teaching. In a sense their contention is true, but not in contradistinction to my contention. The distinction would hardly be worth fighting over were it not that they took up the further ground that only likely to interest the students. Of course the leaders of science those researches should be engaged in in Universities which were can if they choose sell the great birthright of Universities for a mess of fees, but I hope they will not be permitted to do so without protest. What view the democracy take of Universities is of the very last importance with our democratic institutions. and I trust all those who have the welfare of the nation at heart will protest against the Universities being turned into coach-houses. In this connection it is most important to bear in mind the distinction between the functions of Universitie and those of schools and colleges. The function of these latter is primarily to teach those who resort to them. The function the University is primarily to teach mankind. In former days when the means for distributing information were very imperfect. students used to flock from all sides to learn directly from a great mind. Nowadays the great mind distributes his teaching broadcast. In old days the only way to learn what was being draz to advance knowledge was to go to the place where knowledge was being advanced. Nowadays we read the Transactions of c learned societies at home. But at all times the greate men have always held that their primary duty was the discovery of new knowledge, the creation of new ides for all mankind, and not the instruction of the few wh found it convenient to reside in their immediate neighbor hood. Not that I desire to minimize the immense importan of personal influence, it is overwhelming; but it is a question quite beside the one at issue, which is whether the advance knowledge by research and the teaching of the whole nation b the discoveries made is not rather the primary object of Univer sities than the instruction of the few students who gather in ther halls that is the real question at issue between Lord Kelv Sir Geo. Stokes, and myself. Are the Universities to devote t energies of the most advanced intellects of the age to ! instruction of the whole nation, or to the instruction of the lev

whose parents can afford them an, in some places fancy, education that can in the nature of things be only attainable by the rich?

In view of the discussion upon the proposed Teaching University for London it is to be hoped that these things will not be overlooked amid the local questions and rival institutions. It is to be hoped on the one hand that those who will have the privilege of learning in the greatest city in the world will not be deprived of the personal influence of its greatest men by relegating these to some haven of laboratories where no bracing

breath of students shall interfere with the inmates. On the other hand it is to be hoped that London will so far honour itself as not to be content until it sees its University a centre of thought and investigation from which shall radiate new ideas and discoveries to enlighten and benefit the whole nation. Before I close there is a matter of great importance to which I fear sufficient importance is not attached by those who are directing this matter and that is the great objections there are to mixing up Universities and Colleges with examining boards. We here in Trinity College, Dublin, suffer very much from the fact that a considerable number of our students never reside here, but only come over for periodical examinations. We only suffer in one way, while if London adopted this abominable arrangement it would suffer in two ways. We suffer because our degree is much less valued than it would be if all our students were compelled to reside. All our students have not that education got by friction with their fellows and by contact with trained intellects which no examination can test, and which is such a valuable training, and in consequence our degrees are the less valuable. London would suffer in this way, and it is a very serious way too. In addition to this London would suffer from the inordinate importance that would be attached to extern examiners if the University examined London and extern students. So far we have escaped this danger, but it is inevitable in London because the extern element there would be large, influential and organized, while with us it is of little strength. The result would be to perpetuate and intensify that horrible teaching for examinations which is so necessary an evil in the case of the majority of students, but from which the leaders of thought should be exempt. It matters not that the syllabus nor even that the very questions are approved by the professor, if the examination is conducted to any serious extent by an independent mind. The student will seek a coach, who will probably teach him very well indeed, but whose whole view of learning will be of the passing-an-examination type, and who will infect his pupil with this miserable disease. Gradually the professor himself will be involved in the vortex, and the whole University will gradually look upon the passing of examinations as the end of life for students, and this is the acme of coaching and the bathos of education. GEO. FRAS. FITZGERALD.

Trinity College, Dublin, November 25.

The Stars and the Nile.

AFTER reading Mr. J. Norman Lockyer's papers on the connection of the orientation of Egyptian temples with the eliacal rising of certain stars, I was interested to find that a ustom still exists in the neighbourhood of the Second Cataract having a strong resemblance to the old Egyptian

ustom.

The Nuba people of this part foretell the first rise of the le by the heliacal rising of the Pleiades, or as they call it, Turaya." For Sirius they have no special name, calling it erely "the driver" or "follower" of the three stars Prion).

It must be remembered that the first sign of the rise at ady Halfa occurs at the beginning of June, reaching Souan about a week later, but for some days the increase is y slow, and scarcely perceptible except in the readings of Nile gauges.

These Nuba people still preserve in their language many ient Egyptian words, and possibly we may have here a trace he old custom, the Pleiades being taken instead of Sirius account of the earlier date of the rise in the district of the ond Cataract than in Egypt itself.

airo, November 14.

H. G. LYONS, Capt. R.E.

A Palæozoic Ice-Age.

THE account by Dr. Wallace in NATURE (p. 55) of glacial deposits recently discovered in Australia is a most important and welcome addition to our knowledge. But to us the surprising circumstance is that Dr. Wallace appears quite unaware of the fact that this is only an addition to a great series of discoveries, by no means confined to Australia, affording evidence of a Paiæozoic ice-age. That the deposits near Sandhurst are Palæozoic may, in the absence of any indication to the contrary, be assumed, since they are clearly similar in position and character

to the well-known boulder beds of Bacchus Marsh, and these have been correlated with the strata containing ice-borne fragments, amongst the marine beds west of Sydney and also at Wollongong to the southward, and in Queensland to the northward. All these beds have been shown to be upper carboniferous. A good account of the facts known up to 1886 may be found in Mr. R. D. Oldham's paper on the Indian and Australian coal-bearing beds (Rec. Geo. Surv. Ind. xix. p. 39).

It is scarcely necessary to refer to the fact that extensive Paleozoic glacial deposits, of the same age as those of Australia, have been found in several parts of India, some as far within the tropic as lat. 18° N., others in the Salt Range of the Punjab, that the famous Dwyka conglomerates of South Africa are similar and in all probability contemporaneous, and that boulder beds of very possibly the same geological date have been observed in Brazil. We should not have mentioned these but for the fact that the idea of a Palæozoic ice-age is apparently novel to Dr. Wallace. We do not think, however, that the reason why so well-informed a naturalist is unacquainted with geological data long known to many is any mystery. It has become an accepted article of faith amongst most European geologists (there are, of course, exceptions) that no ice-age occurred before the last glacial epoch, just as it is part of the geological creed that the carboniferous flora was of world-wide extension, and as it has become the prevailing belief that the deep oceans have been the same since the consolidation of the earth's crust. Now the discoverers of glacial evidence in the carboniferous beds of India and Australia also assert that the carboniferous flora of those countries differed in toto from that of Europe and resembled the jurassic flora of European regions, and some of them add that the great southern flora of South Africa, India, and Australia must have inhabited a vast continent, part of the area of which is now beneath the depths of the Indian Ocean. Partly from Indian and Australian geologists being regarded as heretics geologically, partly from other causes, the evidence of ice action in India and Australia has been generally ignored. No better proof could be afforded of the fact that European geologists in general have omitted to notice the series of discoveries in the southern hemisphere and in India than the publication of Dr. Wallace's paper.

The glacial evidence as it now stands is extremely interesting and perhaps transcends in importance that of the Pleistocene glacial epoch. For as the effects of the carboniferous ice-age were felt within the present tropics, either the earth's axis of rotation must have shifted considerably, or else the refrigeration of the surface must have been due to a cause distinct from that supplied by the late Mr. Croll's theory, even when supplemented by Sir R. Ball's amendment.

Our own interest in the whole subject is chiefly due to the circumstance that we happened in 1856 to be the first who met with the ancient boulder-bed in India, and suggested that it Australia and South Africa were of course quite independent The discoveries in might be explained by the action of ice. of those in India, but were, we believe, slightly later in date. November 20. W. T. BLANford. HENRY F. BLANFORD.

Geology of Scotland.

MAY I supplement Prof. Green's history of geological mapping in Scotland (NATURE, vol. xlvii. p. 49) by pointing out "A Geologi

that Mr. Cruchley published, on March 23, 1840, cal Map of Scotland by Dr. MacCulloch, F. R. S., &c., published by order of the Lords of the Treasury by S. Arrowsmith, Hydrographer to the King.' This fine map is on the scale of four miles to an inch. From the omission of "the late" before MacCulloch's name, it seems possible that the plates were in course of engraving before his death in 1835.

GRENVILLE A. J. COLE. Royal College of Science for Ireland, Dublin.

British Earthworms.

I ENTIRELY concur with Dr. Hurst's view that the supposed new species, described by the Rev. Hilderic Friend as L. rubescens is in reality Savigny's L. festivus. I may add a further reason for discarding the term L. terrestris, Lin., and substituting L. herculeus, Sav., for our common large worm. Savigny himself used "Enterion terrestre to indicate a worm differing considerably from L. terrestris, Lin., in the position and extent of the clitellum; moreover it belongs to the genus Allolobophora and not to Lumbricus at all.

[ocr errors]

With regard to the second "new" species, A. cambrica, recently described by Mr. Friend, I believe that it is merely a variety of A. chlorotica, Sav.

According to the description it appears to differ from the latter species in three points :-(1) colour; (2) extent of clitellum; (3) number of spermathecæ.

(1) Now, amongst my collection of British worms I find one, of which a water-colour sketch taken from the living specimen closely resembles Mr. Friend's description of the colour of A. cambrica. My notes as to size, habits, &c., agree with his description. I have carefully re-examined my specimen, and find that it agrees perfectly with A. chlorotica; or, in other words, I find that A. chlorotica may vary-as Hoffmeister knew that it did vary so much as to resemble A. mucosa, and I may suggest that it is a mimetic resemblance.

(2) Further, with regard to the clitellum of A. chlorotica ; in the table given by the Rev. Hilderic Friend, it is stated to cover somites 29-36. As a matter of fact the next somite, 37, is nearly always included. This brings A. cambrica, Friend, into harmony with A. chlorotica, Sav.

(3) Thus the only differential character left is the number of spermathecæ; and I cannot agree to the validity of a new species on this single character; several specimens should be examined to settle the point, as variation in this feature is known

to occur.

I take a certain amount of credit to myself for the useful faunistic studies on the earthworms of Great Britain, now being pursued by the Rev. Hilderic Friend, for, if I mistake not, I put him in the way of recognizing their specific characters, when, some years ago, I named for him, with remarks thereon, sundry consignments of some scores of worms which he sent to me for that purpose. WM. BLAXLAND BENHAM. The Dept. of Comparative Anatomy, Oxford, November 21.

Egyptian Figs.

THE accompanying sketch represents an instrument used in Egypt for removing the "eye" or top of the sycomore fig. It is a piece of hoop iron, blunt on one edge and tolerably sharp on the other, and fixed into the end of a stick. The fruit of Ficus sycomorus, or "Egyptian fig," seems to be invariably infested with the insect Sycophaga crassipes, Westw. ; which I am informed by Rev. T. F. Marshall, who has kindly given me the name, is the same insect supposed to effect caprification in Malta, judging from specimens which I sent him. This fig never produces ripe seed in Egypt, though it has been introduced from the earliest times. Not only are the ancient coffins made of the wood, but it was adopted as the sacred "Tree of Life.

a

It probably came from Yemen, where Dr. Schweinfurth saw many seedling trees growing spontaneously. The tree bears three crops per annum, in May, June, and August-September. Boys cut off the top of the figs of the first two crops only. Dr. E. Sickenberger, one of the professors in the School of Medicine, Cairo, informs me that the figs have no pleasant flavour until the operation has been performed:-"They then become very sweet, but remain smaller than when not cut open. The object is to let the insects escape. Those that are left become watery and tasteless, and are full of namoos or svco phaga." In his first description Dr. Sickenberger described the instrument as a kind of thimble made of iron plate

[ocr errors]
[ocr errors]

ending in a spatula like a finger-nail. It is fixed on the thumb of the right hand. The opera ion is only made on fruits which shall be picked up the following day. The day after the operation the fig is quite ripe. The male flowers in those fig、 are all aborted, and the females have never perfect seeds. The figs of the third generation are larger, of an agreeable taste, and sweet-cented: but they are not operated upon, only because in August and September, though the trees are much fuller of fruit than in May and June, the people have so much to do at that time. They are seldom sold, and only eaten by the owners of the trees, or else they are abandoned to the field-mice, birds, and dogs, which latter are very fond of them. These wilg fruits are full of sycophaga."

It will be seen that the instrument he has sent me is of a different shape to the one he describes; and the chief interest lies in the fact that Pliny also describes the process as closely corresponding with this modern method. He even uses a similar term “ nail” (ὄνυχας) : πέπτειν οὐ δύναται ἂν μὴ ἐπικνισθῇ· ἀλλ' ἔχοντες ὄνυχας σιδηροῦς ἐπικνίζουσιν· ἃ δ ̓ ἂν ἐπικνισθῇ, τετάρταια TÉTTETαι (Nat. Hist. xiii. 14). Further, the Prophet Amos describes himself as bōlās siqmīm ; and the authors of the LXX, writing in Alexandria, appear to have understood the expression and translated these words by kvíswv ovkáμiva. This is the same verb as that which Pliny uses; so that it would seem to be pretty certain that Amos performed identically the same operation on the figs as is still done in Egypt at this day. It will be noticed that the idea was to ripen the figs. It does not really do this. because there are no seeds; but it does make the fig sweeter. It also liberates the insects, and without doing this the figs would be uneatable. Jerome is the only author, as far as I know, who alludes to "grubs" being inside the fig. GEORGE HENSLOW.

Iridescent Colours.

THE article "Iridescent Colours" on p. 92 puts me ia mind of a notice which I published thirty years ago, while I lived in the United States. It was entitled "Harmonies of Form and Colour (Stettiner Entom. Zeitung, 1862, PF 412-414), and a portion of it refers to the subject of the above-mentioned article in NATURE, and may be of interest to its readers :

"A fundamental observation, which proves the influence of the intensity of light upon colour, may be made on some insects of metallic coloration, inhabiting a large area from north to south. About six years ago, while in Southern Russia, I took a walk during sunset, and was struck by the brilliancy of some metallic red Chrysomela, abundant in that locality. I found that it was the common C. fastuosa, which I did not recognize at once, because in the environs of St. Petersburg, where I lived at that time, it occurs in its metallic-green variety, with an iridescent blue stripe on each of the elytra. Still farther north it assumes a more violet metallic colour. The same is the case with Chrysomela cerealis and C. graminis. The firs of these species is represented in St. Petersburgh in the be variety (C. ornata, Ahrens), while the typical variety, occurring farther south, has purpli-h-red metallic stripes. It is evideo. therefore that the metallic colouring of these wide-spread species is gradually intensified from north to south, in the order of the colours of the spectrum. We may imagine the area which these beetles occupy, like an immense rainbow, reflectet from their backs, violet in the north, red in the south; th violet perhaps connected in some way with the magnetic phea mena prevailing in the polar regions. The longicorn beet (Callidium violaceum) undergoes the same variation: violet à the north, blue in central Europe." C. R. OSTEN SACKEN Heidelberg, Germany, November 27.

The Afterglow.

THERE has been for three weeks past a very remarkable newal of the afterglow. There is a quite deep secondary glow after the stars are fully out. I should say that no s afterglow has been seen since 1886, or three years after Krakatão eruption. There is also a great extension of the w hazy atmospheric corona around the sun, very marked as around the moon. I am unable, however, to make out an

the pink colour on the outer edge of the haze, which was so ch

[ocr errors][merged small][ocr errors]

66

acteristic of Bishop's Ring," and distinguishable at Honolulu for two years. Apparently there has recently been a great reinforcement added to the material in the upper atmosphere, which produces the afterglows.

Is this owing to the August eruption in Alaska, which is said to have distributed ashes at a distance of 250 miles?

Prof. C. f. Lyons, in charge of tidal observations in Honolulu, reports the period of highest mean tide to have extended itself this year into November, or fourteen months later than the last similar period. The mean sea level is now over ten inches higher than it was last April. It is also somewhat higher than has been shown by any previous tide registers in Honolulu. Mr. Lyons regards this as of special importance, taken in connection with the oscillation of the earth's axis, now established by the combined observations at Berlin and Honolulu. Honolulu, November 8. SERENO E. BISHOP.

OSMOTIC PRESSURE.

OF the various properties which have found a common

explanation in the new theory of solutions, there are none perhaps to which more interest attaches than to osmotic pressure; and although, on account of the experimental difficulties, the observations as yet accumulated on this subject are but scanty, they have so largely contributed to the novel ideas involved in the new theory, that they merit special attention.

Since accounts of osmotic pressure are finding their way into few English text-books, it may be worth while glancing at the main features which have led up to the present state of the question.

It has long been known that if an aqueous solution say, of sugar-be separated from pure water by a piece of animal membrane, that movements of the water and of the sugar take place through the membrane. If the solution be contained in an open vessel, the base of which is composed of membrane, on partially immersing the vessel in water it is easy to see that more water enters the vessel than solution leaves it. The level of liquid within rises above that without the vessel, different pressures being thus set up on opposite sides of the membrane.

To this process wherein currents pass through a membranous septum, the terms "osmosis," "osmose," and "diosmose" have been applied. The last of these is perhaps to be preferred, as it serves to indicate that two currents are involved in the phenomena. Investigations carried out as indicated above were concerned with the measurement of what was termed the "endosmotic equivalent." That is the ratio of the amount of water passing into the solution to the amount of dissolved substance passing in the opposite direction. Consistent measurements of this quantity could not be obtained, however, for it was found that the nature of the membrane exercised a marked influence upon its magnitude. The kind of membrane employed, or, with the same membrane, its thickness or freshness, or even the direction in which water passed through it, was of importance. Thus in illustration of the last point, water passes more readily outwards through eel's-skin, more readily inwards through frog's-skin.

To obtain quantitative relations in this field it thus became essential to eliminate the influence of the membrane, and more recently this end seems to have been attained by the use of membranes artificially prepared. These artificial membranes differ from those of animal origin in the remarkable particular that although they allow water to pass through, they present a barrier to he passage of certain dissolved substances. On this ccount they have been termed semi-permeable memranes, and by their use measurements of osmotic ressure have been made possible.

To carry out such measurements the first point to be olved was to obtain a membrane of sufficient strength.

The substance which has been found to be most satisfactory as a membrane-former is copper ferrocyanide. When aqueous solutions of potassium ferrocyanide and copper sulphate are carefully brought into contact a pellicle of copper ferrocyanide is formed where the two solutions meet. In this condition the pellicle is much

too fragile to sustain even slight differences of pressure; but by the following simple device, employed first of all by W. Pfeffer, satisfactory results have been obtained.

If a cell similar to the ordinary porous pot of a voltaic battery be lowered into a solution of copper sulphate while at the same time a solution of potassium ferrocyanide be poured into its interior, the two solutions meet somewhere within the walls of the cell and deposit a film of copper ferrocyanide. Little diaphragms of membrane are thus produced stretching across the pores of the cell-wall, which furnishes the necessary support, and by taking suitable precautions a membrane may thus be obtained capable of withstanding a pressure of several atmospheres.

The behaviour of a solution when separated from pure solvent by such a semi-permeable membrane differs markedly from what takes place when an animal membrane is employed. In the latter case, at the outset water adds itself to the solution; the level of liquid and the pressure on the solution-side of the membrane thus rise until a maximum pressure-head is attained, which, roughly speaking, is greater the stronger the solution used. Seeing, however, that dissolved substance is continually escaping from the solution through the membrane, as soon as the maximum is reached the pressurehead begins to fall until eventually it vanishes, the levels of liquid on either side of the membrane being the

same.

If, on the other hand, a semi-permeable membrane be employed, as before, a maximum pressure is attained; but since dissolved substance cannot leave the solution, this maximum pressure as well as the concentration of the solution remain constant.

When this constant state of things is established the excess of pressure on the solution-side of the membrane over that on the solvent-side, whatever it may mean, is termed the "osmotic pressure" of the solution. It is therefore customary to reserve the term osmose to phenomena relating to semi-permeable membranes; diosmose being used in cases where, as with animal membranes, dissolved substance as well as solvent can traverse the membrane. It is obvious that when the pressure is established as indicated above, the original concentration of the solution has been altered by the entrance of solvent, and the observed osmotic pressure refers of course to the solution having the final concentration. If, however, we imagine the vessel containing the solution to be closed at the top, a quantity of air being imprisoned over the solution, pressure may be set up by compressing this air, only a small quantity of solvent being allowed to enter. If, further, the air enclosure be tapped by a manometer, measurements of the pressure may be taken, and by making the air enclosure and the volume of the manometer small enough the quantity of solvent entering while pressure is being established may be neglected, the original concentration of the solution remaining practically unaltered. This is the principle of the method employed in measuring osmotic pressure in absolute

units.

The question now arises, "Are these measurements really independent of the nature of the membrane? Has the difficulty which beset the older experiments been overcome?' To this question an immediate answer is for thcoming, for, as pointed out by Prof. Ostwald, it follows from theoretical considerations that if the membrane employed is really semi-permeable, the observed osmotic pressure of a given solution must be the same, no matter of what material the membrane is com

posed. For suppose we have a quantity of solution enclosed in a tube, one end of the tube being closed by a membrane A, the other by a membrane B, and suppose it possible that a pressure P can be developed on the membrane A when it separates the solution from pure water, which is higher than the pressure p similarly developed when B separates the solution from pure water. On immersing the tube in water, the latter will begin to pass through both membranes into the solution. When the pressure p is attained passage through B will stop, but that through A will continue; but as soon as the pressure on the solution rises above p, water will be forced out through B. The pressure P will thus never be attained, water will continuously enter through A, and pass out at B. We will thus have a machine capable of doing an infinite amount of work, which is impossible. Similar reasoning shows that cannot be greater than P; it follows therefore that the pressure developed on each membrane is the same, that the osmotic pressure must be independent of the nature of a truly semi-permeable membrane.

Actual observations are on record in which the osmotic pressure did appear to vary with the membrane employed. A sugar solution, for example, exhibited a much lower osmotic pressure with a membrane of Prussian blue or calcium phosphate than with copper ferrocyanide, From the preceding argument it is concluded, however, that these membranes giving the lower values were not quite firm or not quite impermeable to the dissolved substance; the highest value is thus taken as the measure of the osmotic pressure which is nearest the truth.

On glancing at the results which have been obtained, the first point which strikes one is the extraordinary magnitude of the pressures thus set up. In the case of a 1 per cent. aqueous solution of nitre the pressure attains the value of 2 atmospheres. This value increases with the strength of the solution till at 33 per cent. it is no less than 6 atmospheres, this pressure being the highest which any membrane yet prepared has been able to withstand. With substances like sugar, other things being the same, the pressure is not so great, but in all cases, in order to keep it within workable limits, the solutions employed have to be dilute.

Striking as the results are themselves, their explanation is not less remarkable. The original measurements of osmotic pressure were made with the purpose of elucidating the movement of liquids in plant cells, and naturally the substances examined were such as occur in the vegetable organism-aqueous solutions of sugar, gum, dextrin, and the nitrate, sulphate, and tartrate of potassium. For some years after these observations were made, they lay comparatively unnoticed, until Prof. van't Hoff, of Amsterdam, turned them to a use undreamt of by their discoverer. From a study of the properties of dilute solutions van't Hoff came to the conclusion that the osmotic pressure was due to the bombardment of the molecules of the dissolved substance on the semi-permeable membrane. For when the osmotic pressure is established and equilibrium exists between solvent and solution, in the same time, equal amounts of solvent, must pass in either direction through the membrane and the impacts of the solvent molecules on the membrane will then be equal and opposed on either side, and therefore negligible. On this reasoning the pressure recorded on the manometer is taken to be that exerted by the substance in solution.

On examining the magnitude of the pressure thus attributed to the dissolved substance, in the case of a solution of sugar van't Hoff next showed that it bore the closest resemblance to the pressure of a gas. Indeed, if we calculate the pressure of a gas which at the same temperature contains as many molecules per unit volume as there are molecules of sugar per unit volume of solution, then the pressure of the gas and the osmotic pres

sure are the same. Moreover, on thermodynamical grounds it was established that on the above hypothesis as to the nature of osmotic pressure its magnitude should be quantitatively connected with measurements of other physical properties of solutions, more especially those on the lowering of the vapour-pressure, and of the freezing point of a solvent produced by the presence of dissolved material. In this way a mass of evidence was collected, a general survey of which led to the foundation of the new theory of solutions. On this theory the dissolved substance, if the solution be dilute, is supposed to behave as if it were gaseous, the pressure it exerts-the osmotic pressure-being equal to the pressure which it would exert if it were gasified, and occupying, at the same temperature, a volume equal to the volume of the solution.

Unfortunately measurements of osmotic pressure have only been made on few substances, and only for solutions in water, but on turning to all the available observations to see how they support this novel conclusion, the most superficial examination serves to show that an agreemen: does not exist. Unless in the case of sugar, for no substance of known formula which has yet been investigated does the osmotic pressure agree with the corresponding gaseous pressure. These substances consist of salt solutions, and they invariably give higher osmotic pressures than theory demands. Similar disturbing influences have been observed when other physical properties of these solutions were measured, and to account for the facts an additional hypothesis has been put forward by Dr. Svante Arrhenius.

Salt solutions are electrolytes, they conduct the electric current, and undergo simultaneous chemical decomposttion into their constituent ions. Experiment shows that such electrolytic solutions give high osmotic pressures, more particles appear to bombard the semi-permeable membrane than if the dissolved substance behaved as a gas. The new hypothesis states that this is really the case, the additional number of particles being produced from the breaking up of the dissolved substance. It states that in a solution which can be electrolyzed a portion at least of the dissolved substance exists already decomposed or dissociated into its ions, and that although these ions cannot be separated by diffusion they are so far independent that each can exercise an effect on the semi-permeable membrane.

The extent of this electrolytic dissociation is sup posed to vary with the chemical nature of the dissolve: substance, and to increase with the dilution. In ver dilute solutions it may be complete, the whole of the dis solved substance being supposed to exist in the state a ions.

The second hypothesis gives, therefore, some expl nation why the osmotic pressure of a salt solution greater than that of a non-electrolytic solution of sug it further fixes the limits between which the osmotic pre sure ought to vary in the case of an electrolyte, for lower limit should be that of undissociated gas, the higre should be that of completely dissociated gas, each original molecule having decomposed into as many sc> molecules as there are ions in each molecule of salt.

So far as these limiting conditions go, the facts s port the hypothesis. In all cases the observed osmoti pressure is either equal to one or other of the limits, e lies between them. A closer scrutiny leads, nevertheles to apparent discrepancy. It is evident that a measure. the amount of dissociation can be obtained from osmet pressure observations. For if we divide the observe osmotic pressure by the corresponding pressure of ut dissociated gas we have obviously, if the preced hypotheses are valid, the ratio of the actual number bombarding molecules to the theoretical number had: dissociation occurred. The ratio of these two numbers. is denoted by the letter "," a factor first used by van Hoff. Now, on the new theory, the value of "¿” can li

« PrejšnjaNaprej »