alone to an explanation of the peculiar mode of operation exhibited by the hydriodic salts on different preparations of silver under the influence of light, have thus opened a new and unexpected field of interesting inquiry, which may possibly end in the establishment of the new art of THERMOGRAPHY.

I shall pursue this subject with the same interest by which I have been led forward in my inquiries on Photography since the publication of Mr. Talbot's processes and those of Daguerre. For a few interesting applications and_curious discoveries which I have made I merit not, nor do I seek praise. To every inquirer there is a mine of discovery, of which the few specimens I have gathered on the surface will, I trust, show the richness of the yet buried treasure.

I remain, Gentlemen, yours, &c.

Devonport, July 4, 1840.

ROBERT HUNT.

XXXIX. On the Form of Rutile. By W. H. MILLER, Esq., Professor of Mineralogy in the University of Cambridge.

THE following values of the angles between normals to

the faces of rutile were obtained from two extremely perfect crystals, for which I am indebted to Mr. Brooke. The instrument used as a goniometer was a twelve-inch theodolite. The coincidences of the signals were observed with a telescope having a power of about twelve. Each result is the mean of two observations made with the signals interchanged, in order to eliminate the error arising from the imperfect centring of the crystal. In one the faces p", p" (fig. 1.) gave double images, those in p" being close and ill

defined. The observed values of p' pl" were 65° 33′ 0"-30" -8"; those of pp" 65° 32' 22", or 65° 34′ 26", according as one or the other of the images was made to coincide with the signal seen by direct vision. In the second and more perfect of the two crystals, the observed values of p' p" were 65° 34'24"-36"-51"-27"; those of pp" were 65° 34′ 27"-53" -25"-35". The values of p'p", pp" given by the second crystal, agree very well with each other, and with the larger of the two values of pp" given by the first. The mean of these three, 65° 34′ 32", will probably be very near the truth.

The sphere of projection (fig. 2.) exhibits the poles of all the faces observed by Mohs and Levy, as well as by myself. The symbols of the simple forms are

z is the intersection of the zone circles pp", sp", cr; t is the intersection of the zone circles pl', ce.

Of the crystals above-mentioned, one is a combination of the simple forms having the faces l, g, h, e, p, s; the other of those having the faces g, h, e, u, p, s; t was observed by Levy (Déscription d'une Collection de Mineraux).

In the collection of minerals presented to the University by Professor Whewell, a crystal occurs which is a combination of the forms having the faces h, c, r, p, z, and also others which are combinations of the forms having the faces x, p, and occasionally c, l, g, h, e. Among the latter are several twins (fig. 3.), in which the faces cp, c, p, are all in one zone, and the angle between normals to cc, rather more than 55°. On account of the unevenness of the faces, this angle could not be accurately measured; if the twin plane were parallel to a plane v, the pole of which is the intersection of the zone circles zz", cl, the symbol of v would be (301), cv = 62° 38′4, cc1 = 54° 43′-2. Hence probably the twin plane is parallel to a face of the form {301}. The twins of most usual occurrence are those described by Haidinger (Edinburgh Journal of Science, vol. iii. p. 62.). Mr. Brooke measured the angle between the faces 11, (fig. 4.) of one of these twins in his collection, and found that it agreed perfectly with the supposition that the twin plane was parallel to one of the faces p.

The values of cp, cs, deduced from the observed values of pl, sg, given by W. Phillips in his Mineralogy (third edition), are 32° 45′ and 42° 20′. My determination of these angles differs only about two minutes from the former, and agrees

exactly with the latter. Unfortunately the figures which were intended to express the angles pp", ss" ("a on a over summit = 90°, con cover summit = 109° 47'") are very erroneous, probably through some mistake in transcribing them, for the errors (which remain uncorrected in the fourth edition) are much too large to have been errors of observation.

St. John's College, Aug. 31, 1840.

W. H. MILLER.

XL. Note upon Mr. Griffith's Paper "upon the true Order of Succession of the Older Stratified Rocks near Killarney and Dublin." By CHARLES WILLIAM HAMILTON, F.G.S. &c.*

THERE are a few points in this paper to which I think

it necessary to reply.

I brought forward the result of my own observations with diffidence, because I felt that they had been few and imperfect; but I think they are not useless, and that I have, as I then hoped, "pointed out an interesting field of inquiry, and helped to show that a large portion of Ireland is still a subject for Geological debate;" and besides, I am still convinced that my observations, as far as they went, were not inaccurate, as Mr. Griffith asserts, although his subsequent observations may have shown that some of my conclusions were so.

Mr. Griffith has represented the fault which is observable at Brickeen Island (L. & E. Phil. Mag. vol. xvi. Pl. 2. fig. 2.) as involving an upcast of about three thousand feet, and the beds of chloritic slate, quartz-rock, and old red sandstone as cropping out at a low angle on the south-eastern slopes of Glena mountain, at the base of which the fault is to be seen. Now in this case I am thoroughly convinced that my re

presentation is correct and Mr. Griffith's incorrect; and as it is a point easily accessible to any geologist who may visit Killarney, I do hope that some one will take the trouble of comparing the two observations. I believe that the succession of the strata is in exactly the reversed order, that there is an anticlinal on the summit of Glena mountain, that the strata on the south-eastern slopes dip to the S.E. at an angle approaching very nearly to the perpendicular, and that consequently the slates at the back of Lady Kenmare's cottage are in the highest and not the lowest part of the series there developed.

Mr. Griffith attaches much importance to the difference of strike upon the opposite sides of the fault. I have made a great many careful observations, and although until we get a correct map it would be impossible to lay these down so as to draw any correct conclusions from them, I may observe that the observations taken at the same side of the fault vary quite as much among themselves as they do with those on the other side.

In p. 173 Mr. Griffith says, "it appears to me that Mr. Hamilton is mistaken in separating the old red sandstone from the Devonian system;" but the very quotation he makes explains clearly that I made no such separation, but referred the compact arenaceous rocks overlying the coarse red conglomerate to the upper part of the Devonian system.

ssand

I shall not occupy your pages with other points in which I am still hardy enough to rely upon my own observations in opposition to so high an authority as my friend Mr. Griffith; but I could not leave these unnoticed, because I wish to press upon the attention of geologists the fact that the difficulties of this district have not yet been cleared away, and that Mr. Griffith is premature in referring to the Silurian epoch, that vast depth of sandstone and conglomerate which occurs between the bays of Kenmare and Castlemaine, and the whole section between Foillatarriv and Brandon; I have before expressed my opinion (Journal of the Geological Society of Dublin, vol. i. p. 282.) that this latter section bears the strictest analogy to that between the Bangor quarries and Llyn Schal, and I see no reason for retracting that opinion.

To Mr. Weaver's remarks and insinuations of " an unrestrained indulgence of fancy," &c., I shall not reply; his objections to the possibility of the old red sandstone dipping in one place to the south, and another to the north, seem hardly to require an answer; and the correctness of his observations has been already subjected to a sufficiently rigorous inquiry by Mr. Griffith.

XLI. On the Heat of Vapours and on Astronomical Refractions. By JOHN WILLIAM LUBBOCK, Esq., Treas. R.S. F.R.A.S. and F.L.S., Vice-Chancellor of the University of London, &c.

[Continued from vol. xvi. p. 569.]

On the Vapours of Ether, Alcohol, Petroleum, and Oil of Turpentine.

THE following Table is extracted from a valuable paper

by Dr. Ure in the Phil. Trans. for 1818.*

Table of the pressure of the vapours of æther, alcohol, petroleum or naphtha, and oil of turpentine.