I have lately been much occupied with your subject of Circular crystals, <1> in consequence of having discovered that the remarkable and hitherto unexplained phenomena of Oil of Mace which I described in the Phil. Trans. for 1815 p. 49 <2> are the direct results of circular crystallisation.
I have made my Expts on Circular Crystals with those obtained from the Litho-Xanthate of Ammonia which Dr Dowler of Richmond <3> sent me some years ago; but being anxious to compare the results with those whh you obtained from Boracic Acid dissolved in dilute Phosphoric Acid, I have tried in vain to procure even the shadow of circular crystals by your method. I have tried separate solutions, too, in various degrees of dilution, and with various rapidities of cooling, but all my attempts have been fruitless.
I believe that I have mastered the subject in as far as it is presented in the Lithoxanthate of Ammonia. The great difficulty I have encountered is in explaining the black Circles which separate the crystalline rings which give polarised tints of difft intensities. I was, of course, very anxious to confirm your views as collaborative of my own regarding the cause of double refraction; but I believe that the black Circles are not lines of compensation separating two difft structures, nor are the tints in any radius of the crystal different from those which are due to the different thicknesses of the crystalline matter along that radius.
With regard to the Oil of Mace Phenomena to which nothing analogous had ever been observed, I can not only shew the very same phenomena in circular crystals, but I can also exhibit the circular crystals shewn in Oil of Mace.
Upon receiving your printed notice on the fusion of Nitre, I repeated the expt & find the same results whh you described. This led me to try a great No of other bodies in which I sometimes found the same property. In making these Expts I often find the air bubbles surrounded, by the pressure of the expanded air with circular polarisation like what I found round minute cavities in Diamond & Topaz <4>.
Believe me to be Dear Sir Ever Most Truly yrs
St Leonards College
Decr 18th 1847
Henry Fox Talbot Esqr
&c &c &c
1. See WHFT, ‘Observations on the Optical Phenomena of certain Crystals’, Philosophical Transactions of the Royal Society of London, v. 127, pt 1 (1836); paper received 20 April, read 5 May 1836. See Doc. No: 03305.
2. D. Brewster, ‘Experiments on the depolarisation of Light, as exhibited by various mineral, animal, and vegetable bodies, with a reference of the phenomena to the general principles of Polarisation’, Philosophical Transactions of the Royal Society of London, 1815, pp. 29–53.
3. Dr Thomas Dowler (d. 1855) of Richmond; MD, etymologist and chemical experimenter (also see Doc. No: 03394.
4. D. Brewster, ‘On the modification of the doubly refracting and physical structure of Topaz, by elastic forces emanating from minute cavities’, Philosophical Magazine, v. 31, 1847, pp. 101–104.