248 Dr. Thomson on the [Oct. 



near purity, that very great precautions indeed must be taken 

 before carbonic acid gas can be obtained in a state of greater 

 purity. 



2. Carbonic Oxide. — The carbonic oxide gas upon which the 

 following experiments were made was obtained by mixing toge- 

 ther equal weights of clean iron filings and pounded chalk 

 previously dried by exposure to a red lieat, and exposing the 

 mixture to a strong heat in an iron bottle. The specific gravity 

 of the gas thus obtained was as follows : 



By first trial ] .9094 



By second trial 1-9700 



By third trial 1-9700 



Mean = 0-9698 



- We have two sets of experiments to determine the specific 

 gravity of this gas, one by Clement and Desormes, and another 

 by Cruikshanks ; both made soon after the discovery of the gas ; 

 and in neither of them, as is obvious from the result of the expe- 

 riments, was the gas operated upon pure. The specific gravity 

 of this gas, according to 



Clement and Desormes, is 0-9409 



Cruikshanks 0-9569 



These numbers are considerably lower than ours ; but as the 

 gases by means of which they were determined were certainly 

 impure, it seems unnecessary to make any observations upon the 

 specific gravities deduced from them. 'Even our gas, though 

 very great precautions were taken in preparing it, was not per- 

 fectly pure. If the chalk be not absolutely free from water, 

 which IS not easily accomphshed, some hydrogen gas is always 

 evolved, which, acting upon the carbonic acid gas, which is 

 driven off along with the carbonic oxide, seems to give origin 

 to a portion of carburetted hydrogen gas. But it is easy to 

 demonstrate from other considerations what the specific gravity 

 of pure carbonic oxide must be. 



It has been sufficiently ascertained by experiment that when 

 one volume of pure carbonic oxide gas is mixed with half a 

 volume of oxygen gas, and an electrical spark passed through 

 the mixture, the two gases unite, and form exactly one volume 

 of carbonic acid gas. It is obvious from this, that if from the 

 specific gravity of carbonic acid gas we subtract half the speci- 

 fic gravity of oxygen gas, the remainder must give us the 

 .specific gravity of carbonic oxide gas. 



Sp. gr. of carbonic acid gas = 1-5277 



Half sp. gr. of oxygen gas = 0-5555 



Sp. gr. of carbonic oxide gas .... — 0-9722 



