328 Royal Society .— 



question. The numbers obtained on analysis agree very well with 

 the formula of pyrotartaric aeid ; I got 44'G instead of 45*4 per cent, 

 of carbon. It had also all the properties ascribed to this acid by 

 Pelouze and Arppe. The crystals were colourless, and very soluble 

 in water, alcohol, and ether. It had an agreeable acid taste. It 

 became semi-fluid at 100° Cent., and melted completely a few 

 degrees above that temperature. Long-continued ebullition in 

 a glass tube converted it into an oil, which was insoluble in cold 

 water, and no longer affected litmus-paper, but which gradually 

 dissolved in hot water, recovering at the same time its acid reaction. 

 The following equation will explain the reaction which gives birth 

 to this acid : 



C 6 H 6 Cy 2 +2HCl-}-8HO = C 10 H 8 O 8 + 2(NH 4 Cl). 



It is highly probable that there exists a series of isomeric acids 

 running parallel to these, which may be obtained by similar pro- 

 cesses from the diatomic radicals contained in the aldehydes. Thus 

 from cyanide of ethylidene t (C 4 H 4 Cy 2 ) we may hope to get an 

 isomer of succinic acid. 



I propose to continue my researches in this direction, and to ex- 

 tend them to the cyanides of the triatomic radicals. 



May 2. — Major-General Sabine, R.A., Treasurer and Vice- 

 President, in the Chair. 



The following communication was read : — 



" On Internal Radiation in Uniaxal Crystals.' , By Balfour Stewart, 

 Esq., A.M. 



The well-known theory of exchanges, which was proposed by the 

 late Prof. Prevost of Geneva, is built upon the fact that a substance 

 placed anywhere within an enclosure of a constant temperature will 

 ultimately attain the temperature of the enclosure. 



In his theory M. Prevost supposes that a constant, mutual, and 

 equal interchange of radiant heat takes place between the body and 

 the enclosure which surrounds it, so that, receiving back precisely 

 that heat which it gives away, the former is enabled to remain «t a 

 constant temperature. 



With respect to this radiation, which is thus supposed to be con- 

 stantly taking place between substances at the same temperature, it 

 had until lately been conceived of as proceeding mainly, if not en- 

 tirely, from the surface of bodies — a very thin film or plate of any 

 substance being supposed to furnish the maximum amount of radia- 

 tion which that substance was capable of affording. 



It lately occurred to the author of this paper, reasoning from the 

 theory of exchanges, that mere surface radiation is not sufficient to 

 account for the equilibrium of temperature which exists between a 

 body and the enclosure which surrounds it. 



These theoretical conclusions have been amply verified by experi- 

 ment, and the subject has been discussed in a paper published in 

 the * Transactions of the Royal Society of Edinburgh ' for the year 

 1858. As the chain of reasoning by which this fact is deduced 



