254 Dr. Thomson on the [Oct. 



having" one atom less of carbon. The gaseous constituents of 

 carburetted hydrogen are condensed into one-third of their bulk, 

 and those of olefiant gas into one-fourth of their bulk, before 

 combination. This demonstrates the great affinity that exists 

 between carbon and hydrogen, and the intimacy of the union 

 which they form. 



5. Ci/anogen. — This gas, which was discovered by Gay- 

 Lussac, is evolved when dry prussiate of mercury is exposed to 

 heat in a small retort. I have not succeeded well in obtaining 

 this gas by means of an Argand's lamp ; but when a few pieces 

 of red-hot" charcoal are applied to the bottom of the retort, the 

 gas comes over in great abundance. It must be received over 

 mercury ; for water absorbs it too copiously to allow us to collect 

 it in sufficient purity for experiment over that liquid. A suffi- 

 cient quantity of it having been collected, and care having been 

 taken to exclude all common air as completely as possible, its 

 specific gravity was found to be as follows : 



By first trial VSOSg 



By second trial 1-8040 



Mean = 1-80395 



The only person who preceded us in taking the specific gra- 

 vity of this gas is Gay-Lussac, who states it at 1-8064. 



Cray-Lussac's very ingenious and satisfactory analysis of this 

 gas puts it in our power to determine its true specific gravity from 

 the data contained in the preceding part of this essay. One 

 volume of it for complete combustion requires two volumes of 

 oxyo'en gas. After combustion, there remain two volumes of 

 carbonic acid gas and one volume of azotic gas. It is obvious 

 from this that it is composed of 



2 volumes carbon vapour 1^^^^^^^^^^ j^^^^ ^^^^ ^^^^^^^ 

 1 volume azotic gas .-...} 



Consequently its specific gravity must be equal to twice the 

 specific gravity of carbon vapour + the specific gravity of 

 azotic sas. 



Sp. gr. of carbon .. = 0-4166 and 0-4166 x 2 = 0-8333 

 azotic gas = 0-9722 = 0-9722 



1-8055 



Thus we see that the specific gravity of pure cyanogen is 



1-8055. Our result is -j-tVt''^^ P'^^*' below the truth. The gas on 

 which we experimented must have been a mixture of 1127-4 

 volumes of pure cyanogen and one volume of air. Now when 

 we collect gases over mercury, it is exceedingly difficult to 

 obtain them free from some little mixture of common air, owing 



