1820.] Specific Gravity of Gases. 169 



We have 0*0694 x 16 = Mill, the sp. gr. of oxygen gas 

 0-0694 X 14 = 0-9722, the sp. gr. of azotic gas 



3. The very disagreeable smell of chlorine and its corrosive 

 properties prevented chemists for a good many years from 

 attempting to ascertain the specific gravity cTf chlorine gas. 

 Sir Humphry Davy, and Gay-Lussac and Thenard were as far as 

 I know the first chemists who made direct experiments on the 

 subject. The following were the results which they obtairxed : 



Davy 2-395 



Gay-Lussac 2-470 



Dr. Prout, in the paper above referred to, has shown from the 

 specific gravity of muriatic acid gas, and from the known fact 

 that it is composed of one volume of chlorine gas and one volume 

 of hydrogen gas united together without any change of bulk that 

 the true specific gravity of chlorine gas must be 2-500. Thus 

 we see that with regard to chlorine gas the very reverse has 

 happened that took place with respect to hydrogen gas. Every 

 succeeding experimenter found it heavier than his predecessor: 

 nor is it difllcult to see the reason of this : it is owing to the 

 common air with which the chlorine had been mixed. The 

 chlorine on which Davy made his experiments must have con- 

 tained seven per cent, of common air; for, supposing the specific 

 gravity of chlorine to be 2-5, and that of air to be 1, then the 

 specific gravity of a mixture of 93 chlorine + 7 air would be 

 2-395, which is the number obtained by Davy. The chlorine 

 gas examined by Gay-Lussac and Thenard was a mixture of 99 

 volumes of pure chlorine and 1 volume of common air ; for the 

 specific gravity of such a mixture would be 2-47. Being aware 

 of the som-ce of inaccuracy in preceding experiments, our busi- 

 ness in the experiments made in my laboratory was to endeavour 

 to procure chlorine gas perfectly pure. This was accomplished 

 by preparing the gas in a pretty large quantity, and not begin- 

 ning to collect it till it had passed over so long that all the com- 

 mon air was expelled. It is obvious, hov.ever, that this will not 

 be accurately accomphshed unless the apparatus is arranged in 

 such a way that the air can make its escape at the upper extre- 

 mity of the vessel in proportion as the chlorine occupies the 

 lower portion of it. A good deal of practice, and every possible 

 attention, enabled us to succeed completely. I have reason to 

 believe that the chlorine whose specific gravity we took was 

 perfectly pure. Three different trials gave the following results: 



By first trial 2-500 



By second trial 2*500 



By third trial 2-500 



Thus all these es^periments exactly coincide with each other ; 

 and what is slill more satisfactory, they are all the very specific 



