On a Property of the Caustic by Refraction of the Circle. 427 



also observed white vapours which coloured concentrated sul- 

 phuric acid brown, and found even some drops of a green tough 

 oil. But he did not follow the subject further. Marchand, on 

 the contrary, conducted the products of decomposition directly 

 over oxide of copper, and determined them as carbonic acid and 

 water, but could perceive nothing of the formation of tar. 



It might indeed be assumed, that, considering the extensive 

 application of this gas, the generation of tar from it must have 

 been often obseryed. The reason why this has not been the 

 case is, perhaps, to be referred partly to the circumstance that 

 tar is formed at a red heat only, and at higher temperatures 

 furnishes only carbon and hydrogen; partly to the fact that 

 coal-gas itself always contains a slight admixture of the vapour 

 of tar, so that even when fresh tar is generated at a red heat it 

 escapes observation. 



The origin of tar from olefiant gas shows that this is much 

 more closely allied to the organic compounds, consisting of 

 carbon, hydrogen and oxygen, particularly to those which at a 

 high temperature form empyreumatic oils or tars, than marsh- 

 gas. The easier decomposition of olefiant gas is also the reason 

 why it enters into composition with other bodies, as chlorine and 

 anhydrous sulphuric acid, more readily than marsh-gas. 



As regards the fabrication of coal-gas, the experiments lead 

 to the conclusion that tar, which always appears in company 

 with this gas, is formed in two different ways ; partly by the 

 decomposition of the olefiant gas already generated, partly at the 

 same time as the olefiant gas, directly from the substance of the 

 coal. For even if the coal were not suited to the formation of 

 olefiant gas, it would, notwithstanding, like most vegetable sub- 

 stances, as wood, cellulose, turf, sugar, and many others, form a 

 tar or empyreumatic oil. This latter portion contains nitrogen, 

 inasmuch as coal contains it, and furnishes the compounds which 

 have recently grown to such importance, aniline, leucoline, &c. 

 The tar produced by the decomposition of olefiant gas is free 

 from nitrogen, and furnishes principally naphthaline. 



LXVI. On a Property of the Caustic by Refraction of the Circle. 

 By A. Cayley, Esq* 



MST. LAURENT has shown (Gergonne, vol. xviii. p. 1), 

 • that in certain cases the caustic by refraction of a circle 

 is identical with the caustic of reflexion of a circle (the reflecting 

 circle and radiant point being, of course, properly chosen), and 

 a very elegant demonstration of M. St. Laurent's theorems is 



* Communicated by the Author. 

 2F2 



