(8.) When the plate is very thin, another set of appearances presents itself.

On splitting a piece of mica to such a tenuity that only a few indistinct broad bands were barely visible with a flintglass prism, I observed at the same time another set of very fine but extremely faint bands, evidently independent of the former.

(9.) When the film of mica was still thinner, the broad bands ceased to appear altogether, and only the fine set were visible. To show these bands the films must be SO thin as to be nearly iridescent: it is difficult to succeed in tearing them off sufficiently fine. I have sometimes used drops of water between glass plates pressed hard together. These bands are always very faint; but they are somewhat more conspicuous with prisms of the more dispersive oils, and always require a strong light to be seen.

(10.) There is, however, a more remarkable circumstance connected with this set of bands; they continue to be formed when the edge of the thin film is towards the thicker side of the prism.

(11.) In pursuing the theoretical explanation, we have to consider the conditions which may affect the rays situated towards the opposite sides of those primary homogeneous pencils, into which the incident beam of light is separated, and which converge in the eye to form the several points in the spectrum, both in the case of the prism, and of interference from grooves or gratings.

Now in either case a distinction of this kind is deducible from the wave-theory, on comparing the length of undulatory route of the two extreme rays of any primary pencil; from which it appears that one of these rays is always more retarded than the other, as well in the prismatic as in the interference spectrum: that side of the pencil which is previously the least retarded, being that to which the plate is applied in the original form of the experiment. This distinction, combined with the general principles of explanation at first referred to, appear to me not only sufficiently to account for the ordinary phænomena, but in my modification of the experiment to assign a reason why a similar effect should be produced on the oppo

site side.

(12.) With regard to the mathematical investigation, in the case of the prism, without going into a formal discussion, it is sufficient to observe, that on the principles of mathematical optics, when a diverging pencil of homogeneous light is refracted through a prism in the position of minimum deviation,

the emergent pencil will originate from a geometrical focus, which is not a single point, but a caustic, whose convexity is towards the edge of the prism. Hence, on the principles of the wave-theory, it follows that the side of the pencil which lies towards the edge of the prism is that which undergoes less retardation, or has the shorter undulatory route; this difference varies slightly for the different primary rays.

I had arrived at this conclusion by a different approximate method, when in some correspondence the Astronomer Royal pointed out to me the above view of the problem, as connected with his investigations " on the light in the neighbourhood of a caustic." (Camb. Trans., vol. vi. part 3*.)

(13.) With respect to the interference-spectrum, we have only to follow out the investigation given in Mr. Airy's tract (Arts. 80, 83.) (as that gentleman has suggested to me) in the following manner.

Taking the focus as the origin and the axis of the objectglass as the axis of x, let zy be the coordinates of any point in the wave, ab those of a point in the focal image on the same side of the axis, then the radius of the wave being c, we have

c2 = x2 + y2;

and expanding and neglecting powers of y above the third, we find

The distance g from xy to ab will be

g = √((x-a)2 + (y-b)2) ;

here performing the various expansions, and for brevity writing e2 = (c-a)2 + b2, we at length obtain (going to the

third power of y)

The terms involving the second power of y have the same value on each side of the axis, and those depending on the third power are found to be

[* See Lond. and Edin. Phil. Mag. vol. xii. p. 452.-EDIT.]

Here, if, (which is the case, the image being form

C

ed in the focus, so that in fact we might assume a = 0,) it follows that when b and y have the same sign, this expression will be negative; that is, for the ray which has b and y on the same side, or is nearest the axis after passing the focus, the route will be the shortest. The difference will be very small, and will vary slightly for the different rays of the spectrum.

(14.) This difference of retardation in the several rays of each primary pencil, combined with the obvious principle laid down by Mr. F. Talbot, appears to me to supply an explanation of the phænomena.

The whole effect in these experiments is made up of two parts, the original retardation, and that superinduced by the plate. If the previously least retarded ray be intercepted, we take the difference, if the most retarded, the sum of the two effects.

When we apply the plate, the whole resulting retardation may fall within the limits, (before mentioned, § 6.) or not, according to the magnitude of the two retardations, and according as we take their sum or difference. If it be beyond the limits for one portion of the pencil, it may be within them for another.

In general, in the original form of the experiment, that is, for plates of ordinary thickness, the difference falls within the limits, though the sum is beyond them, for all portions of the pencil. But with a very thin plate, the sum may also be within the limits for those parts of the pencil whose difference of retardation is small: Or, in other words, with plates of a certain thickness, the retardation is too great to give bands with any portion of the pencils, when the plate is applied to the previously most retarded side: but it will give bands with some portion when applied to the previously least retarded side.

On the other hand, if an extremely thin plate be applied to the most retarded side, it will still give bands with one portion of the pencils, as well as when applied to the least retarded side with other portions.

Oxford, July 5, 1840.

XIV. On the Potatoe Spirit Oil of the French Chemists. By JAMES APJOHN, M.D., M.R.I.A., Professor of Chemistry in the Royal College of Surgeons, Dublin*.

IN December 1838, I received from my friend Mr. Scanlan

a specimen of an oily fluid which had been given him by Mr. Bowerbank, an eminent London rectifier, and which the latter gentleman had found in small quantity in the faints or weak spirit drawn off towards the close of the rectification of common whisky. Shortly previous to this time, Mr. Coffey, the inventor of the celebrated patent still, had observed the same substance at the extensive distillery of Sir Felix Booth; and upon coming over to Dublin, and visiting the establishment of Mr. Busby at Blockpitts in this city, Mr. Scanlan had the satisfaction of recognizing this same oil in the faint vessel, constituting a thin stratum resting upon the surface of the remainder of the fluid.

The oil obtained from Mr. Busby's concern had a reddishbrown colour, owing to dissolved vegetable matter, and its specific gravity was 8401, that of the faints on which it rested being 9269. Shaken in a graduated tube with an equal bulk of water, its volume was reduced 20 per cent., and the water upon distillation yielded alcohol. To insulate the oil, therefore, the following method was adopted.

The fluid obtained from the faint receiver was first washed with an equal bulk of water; then shaken in a bottle with an equal weight of pulverized and anhydrous carbonate of potash, and finally distilled from a glass retort, the condensation being effected by Liebig's tube refrigeratory. It began to boil at 262°, after which the temperature rose gradually until it became 268°, at which it continued until the whole of the oil was nearly over. The fluid first drawn off was set apart, as still containing alcohol, and that alone reserved for further purification which distilled over at 268°. This portion was redistilled. The ebullition commenced a little over 267°, and in less than a minute rose to 268°, at which point it continued until the rectification was nearly completed. The first and last portions being rejected, the middle portion, or that which came over at 268°, was set apart for experiment.

The oil thus procured is a perfectly colourless liquid, destitute of all viscidity. The specific gravity is 8138, and, as has been already observed, it boils steadily at 268°; cooled to -6° it does not congeal. With rectified spirit it is miscible in all proportions, its specific gravity being thus augmented, and its boiling point lowered.. It is immiscible with water, but nevertheless when agitated with this liquid, it absorbs an appreciable quantity of it. It has a pungent and peculiar odour, and a sharp biting taste, somewhat similar to that of the oil of cloves. When gently heated it readily takes fire upon approaching to it a lighted taper, and burns with a clear flame unaccompanied by smoke. It is an excellent solvent for the fats, and also for resinous substances. Camphor, for example, is readily dissolved by it; and the same may be said even of copal, if a gentle heat be applied. Potash is taken up by it in considerable quantity, oil of vitriol gives it a crimson colour. To determine its composition the following experiments were made:

(1.) 4.24 grains of the oil burned in the usual manner with oxide of copper gave of water 5.06 grains, and of carbonic acid 10.42 grains.

(2.) 7.71 grains gave of water 9.22 grains, and of carbonic acid 19.12 grains.

(3.) 6.63 grains gave of water 8.05 grains, and of carbonic

acid 16-26 grains.

The following are the results deducible from these experi

ments:

(1.)

(2).

Carbon......... 67.96

68.59

(3.)

67.84

13.48

18.68

100

The means of the numbers yielded by the three experiments are given underneath in column (a.). The numbers in column (b.) are the quotients of the corresponding ones in (a.) divided by the respective atomic weights of carbon, hydrogen, and oxygen, and those in column (c.) are others in the same ratio with the quotients.

The inspection of the latter shows that the most probable formula for the oil is C, H, O1, which would give the follow

ing parts per cent.

Carbon

......

.....

68.60

Hydrogen......... 13.45
Oxygen............