My dear Sir,
I ought to have answered your last letter sooner, but I have been unusually busy for the last week. Altho’ I have not yet published in any shape my discovery respecting the Nitrous Acid Gas spectrum, yet there is no reason now why I should avail myself any longer of your kind forbearance in publishing the phenomenon which you have discovered. My Paper <1> will be read at the Royal Society of Edinr <2> on the 15th of April; and if the phenomenon you refer to is intimately connected with my paper, I should be glad to notice it in it.
Your paper <3> is exceedingly interesting & important & will appear in next No of the Journal. I should be much obliged to you if you could give in test tubes or in small bottles some of your some of your Solutions with Colour, such as the sulphate of Molybdenum <4> &c.
The crystallisation of the Muriate of Soda in the form of the Pyramidal Dodecahedron, (which Hauy <5> erroneously made the primitive form of Quartz and Phosphate of Lead) is highly curious, & merits further Investigation. Bowman gives this figure as the primitive form of Sulphate of Potash; but I have shewn that it is a compound crystal made up of Rhomboidal prisms, & Mr Brooke <6> long afterwards did the same thing. I fear your crystals are too minute to produce perceptible tints unless when they have high Double refraction. It is however worth your while to try if the Pyramido-Dodecahedral Muriate of Soda has Double Refraction, for I am confident it must have it if this is really its form. This observation you will find it very easy to make, thro' inclined faces even tho' the polarised tint thro' parallel faces is not visible.
As you can command the Sulphate of Nickel to crystallise in different forms, & thus give it one or two axes of double refraction at pleasure, pray try the Sulphate of Potash which crystallises in Rhomboidal Prisms with two axes of Double Refraction; but which I have often found in regular hexaedral prisms with one axis of Double Refraction. In this case however, I believe the presence of a little Iron extinguished one axis & changed the form. I had, or have yet perhaps, a crystal in the form of the Pyramidal Dodecahedron, with a Rhomboidal prism of the same salt attached to one of the Pyramids.
You ask how this variety of form is consistent with the primitive form. I think an answer to this question may be obtained from the principles I have laid down in the Phil. Trans. <7> 1830 p. 87 – Muriate of Soda for Example has three equal rectangular axes of crystallisation & consequently D. Refraction A, B & C. If its solution is perfectly pure its particles will form a perfect cube. But if there is any impurity which may attach itself to any axis, suppose C, or affect that axis in any way so as to increase or diminish its attraction, then they will form a solid as if the particles consisted of two equal axes A, B & one unequal axis C; & the resulting crystal will belong to the Uniaxal system viz the Rhomb, or the Square based prism. If the impurities affect both B and C and do it unequally, then the new form will be prismatic. On the other hand if A, B & C are three unequal axes, one or two of them may be rendered equal by the causes I have mentioned, and the prismatic form will pass into the uniaxal, or into the tesselar.
I will write you a separate letter on the Lines in the Spectrum, as I have neither time nor room at present.
In an Article <8> in the Quarterly Review Oct. 1830 vol XLIII p. 306, I have given some general views on the Patent Laws, &
in the Art. Patents in the Ed. Encyclopædia <9> tho’ not written by me, embodies many of my opinions. Mr Godsons <10> Bill contains many net Improvements in the law; but it is monstrous to entertain the Idea of keeping the Fees as they are. They shd be totally abolished. Most Patentees take a patent only for England which costs about £120 & yet Mr Godson proposes to make them all pay above £200, as half of the present fees for all the three kingdoms.
I am My Dear Sir Ever Most Truly Yours
H.F. Talbot Esq
1. ‘Observations on the Lines of the Solar Spectrum, and on those produced by the Earth’s Atmosphere, and by the Action of Nitrous Gas’, Transactions of the Royal Society of Edinburgh, v. 12, 1834, pp. 519–530.
2. Royal Society of Edinburgh.
3. W. H. F. Talbot, ‘Remarks on the Chemical Changes of Colour’, Philosophical Magazine, s. 3 v. 3 no. 1, May 1833, pp. 359–360.
5. René Just Haüy (1743–1822).
6. Probably Henry James Brooke (1771–1857), crystallographer.
7. D. Brewster, ‘On the production of regular double refraction in the molecules of bodies by simple pressure; with observations on the origin of the doubly refracting structure’, Philosophical Transactions of the Royal Society of London, 1830, pp. 87–96.
8. D. Brewster, ‘Review of Charles Babbage’s Reflexions on the Decline of Science in England and on some of its Causes’, The Quarterly Review, v. 43 no. 86 (October 1830, pp. 305–342).
9. D. Brewster, ed., The Edinburgh Encyclopædia (Edinburgh: Blackwood [etc.], 1813–1830).
10. Richard Godson (1797–1849), MP for Kidderminster 1832–35. On 19 February 1833 he sought leave to bring in a Bill ‘to amend the laws respecting Letters-patent for inventions …’.