Prof. Kopp on unusual Vapour Densities. 519 



which those phsenomena will be explained where velocity enters 

 as influencing the result of chemical affinity. 



In the above communication the view has been again raised 

 and furnished with new grounds by Deville^ that a body may 

 decompose at a higher temperature, and at a lower temperature 

 be again formed out of the elements into which it had decom- 

 posed, — so that, knowing the starting-point and ultimate results 

 of the action of heat upon a substance, no change would appear 

 to have taken place. Kopp* points out that, in the fact that 

 this is possible, and probably is moi-e frequent than is generally 

 supposed, an explanation may be found of the unusual condensa- 

 tion shown by a number of substances in the vaporous condition. 



Bodies usually correspond to a condensation to one, two, or four 

 volumes. But there are compounds which, from the determina- 

 tions of their vapour-densities, show condensations differing from 

 these ; and some of them, which may be considered as deter- 

 mined with certainty, are as if they had been decomposed, at the 

 temperature at which the vapour-density was taken, into sub- 

 stances out of which they could be again formed at lower tem- 

 peratures. 



For the haloid compounds of ammonium, NH'' CI, NH'*Cy, 

 analogy with HCl would lead us to expect a vapour-density cor- 

 responding to a condensation to 4 volumes. Bineau found that 

 their vapour-density corresponded to a condensation to 8 volumes; 

 and it is probable that at their volatilizing-point they decompose 

 into two substances whose space together occupies 8 volumes — 

 into NH^= 4 vols., and HCl or HCy, also = 4 vols. The same 

 is probably the case with the compounds PH''Br and PH'*C1. 



Analogy with HS would lead us to expect for Nil'* S a con- 

 densation to 2 volumes. Bineau's determination gave 6 vols., 

 just as if the compound had decomposed into NH^= 4 vols., 

 and HS= 2 vols. The vapour-density of NH^ S, HS ought to 

 correspond to 4 vols. Bineau found it to be 8 vols. Probably 

 it decomposes into NH''= 4 vols., and into 2HS= 4 vols. 



Cahours found that PCP showed, at temperatures which lie so 

 far above the boiling-point of the body that its vapour density 

 was constant, a condensation to 8 vols. This might be a decom- 

 position into PC1^= 4 vols, and 2C1= 4 vols. 



Carbamate of ammonia, NH^O, NH^C^O^ (the body gene- 

 rally termed anhydrous carbonate of ammonia, 2NH'', 2C0'^), 

 was found by Rose and Bineau to show a condensation to 12 

 vols., which would be as if it had split up into 2Nn^= 8 vols., 

 and 2C02= 4 vols. 



Bineau had suggested the above explanation for carbamate of 

 * Liebi{<'s Annulvn, Miirch 1858. 



