Sir David Brewster on the Structure of Topaz, 101 



contain any notable quantity of the latter, but, until the expe- 

 riments ai'e carried further, it must remain a question whether 

 the oxygen results from a small quantity of water contained 

 in the liquid, the hydrogen combining with the liquid itself, or 

 from a decomposition similar to that of the peroxides. The 

 experiments certainly add a new and striking analogy to those 

 already known to exist between the peroxides and the halo- 

 gens, l3ut they do not, as far as I have hitherto carried them, 

 necessarily prove analogy of composition. 



In conclusion, I would call attention to a point which I 

 omitted to notice in my original paper, viz. the explanation 

 afforded by the results contained in it of the hitherto myste- 

 rious phaenomena of the non-polar decomposition of water by 

 electrical discharges, as in the experiments of Pearson and 

 Wollaston. This class of decompositions may now be car- 

 ried much further. With the exception of fused metals, I 

 know of no liquid, which, when exposed to intense heat such 

 as that given by the electric spark, the voltaic arc, or incan- 

 descent platinum, does not give off permanent gas; phos- 

 phorus, sulphur, acids, hydrocarbons, water, salts, bromine 

 and chloride of iodine, all yield gaseous matter. 



Viewing these effects simply as facts, and without entering 

 on any theoretical explanations or speculations, I cannot but 

 think that there is a remarkable generality pertaining to them 

 worthy of the most careful attention. 



The apparatus I have described, particularly that repre- 

 sented by fig. 5, and the numerous applications of voltaic 

 ignition which will occur to those who duly consider the sub- 

 ject, promise, I venture to believe, new methods and powers 

 of investigating the molecular constitution of matter, and will, 

 1 trust, lead to many novel and important results. 



Nov. 10, 1846. 



XIX. On the Modification of the Doubly Refracting and Phy- 

 sical Stnicture of Topaz, by Elastic Forces emanating from 

 Minute Cavities. By Sir David Bkewster, K.H.i D.C.L., 

 F.B.S., and F.P.R.S. Edin.'^ 



[With a Plate.] 



WHILE examining, in polarized light, the form and 

 structure of the numerous crystals which I had dis- 

 covered in the fluid cavities of topaz, my attention was par- 

 ticularly called to certain optical phaenomena exhibited in 

 other parts of the specimen. These phaenomena, when first 



* Read before the Royal Society of Edinburgh on the 20th of January 

 1845, and published in their Transactions, vol. xvi. part l.p. 7. 



