174 Df' Thomson on the [Sept. 



The mean of the two gives 2- 40 15 for the specific gravity of 

 protoxide of chlorine. This result differs a good deal from the 

 theoretical number. The deviation indeed is not far short of -^th 

 part. Though this may appear an enormous error to the mere 

 speculative chemist, I am persuaded that it will not in the least 

 surprise any jjerson who has been in the habit of making expe- 

 riments on this gas. The slowness with which the gas comes 

 over, and the small quantity to which we are limited by the 

 necessity of collecting it over mercury, render it peculiarly 

 difficult to exclude common air completely. It was to the pre- 

 sence of a certain portion of air I am persuaded that the devia- 

 tion in our experiments was owing ; and my stock of chlorate of 

 potash happened to be exhausted, which put it out of our power 

 to repeat our experiments at the time. A mixture of rather less 

 than three per cent, of air with the protoxide of chlorine subjected 

 to experiment v.ould have reduceit the specific gravity to 2*4015, 

 which was the number we obtained. I do not, therefore, consi- 

 der the deviation of my experiments from the theoretical result 

 as affording any presumption that the theoretical result is erro- 

 neous. Indeed I was rather gratified to obtain so near an 

 approximation as I did, considering the great difficulty of 

 collecting this gas in a state of absolute purity. 



3. The only known gaseous compound of azote and hydrogen 

 is ammonia. This gas has been long known, and has been 

 shown by decisive experiments to be a compomid of one volume 

 of azotic gas and three volumes of hydrogen gas condensed into 

 two volumes. Hence we obtain its specific gravity by adding 

 together the specific gravity of azotic gas and three times the 

 specific gravity of hydrogen gas, and dividing the sum by 2. 



Specific gravity of azotic gas = 0-9722 



hydrogen gas.. = 0-0694 x 3 = 0-2083 



2)1-1805 



0-59027 



According to this determination, the true specific gravity of 

 ammoniacal gas is 0-59027. 



In consequence of the opinion advanced by Sir Humphry Davy 

 that ammonia contains oxygen as a constituent, the specific 

 gravity of this gas has been" determined with more care than, 

 perhaps, has been bestowed upon any other gas. The following 

 are the results obtained : 



. Kirvvau 0-600* 



Davy 0-5505t 



Allan and Pepys 0-6022$ 



* On Plilogiston. p. 28. + Researches, p. 565. 



i Phil. Trans. 1808. 



