172 Dr. Thomson on the [Sept. 



gas; or, in other words, we obtain it by adding together the specific 

 gravities of oxygen gas and of azotic gas, and dividing the sumby 2. 



Specific gravity of oxygen gas I'llll 



azotic gas 0-9722 



2)2-0833 



1-04Tg6 



The first person who attempted to ascertain the specific gra- 

 vity of this gas was Mr. Kirvvan. He found its specific gravity 

 1*1887.* This greatly exceeds the theoretical specific gravity; 

 but Mr. Kirwan was not remarkable for the accuracy of his 

 experiments ; and at that time it was customary to obtain nitrous 

 gas by the application of heat to a mixture of copper filings and 

 dilute nitric acid. Now when the process is conducted in this 

 way some protoxide of azote is apt to be evolved, and to mix 

 itself with the nitrous gas. This is more especially the case 

 when the copper fihngs contain a mixture of "brass, which, as far 

 as my own experience goes, is almost always the case, unless the 

 copper filings be prepared on purpose for the experiment. A 

 mixture of yti-3 volumes of nitrous gas and 3-5 volumes of pro- 

 toxide of azote would have the specific gravity found by Mr. 

 Kirwan, 



The nitrous gas upon which we made our experiments in my 

 laboratory was prepared by dissolving slips of pure copper in 

 dilute nitric acid. No heat was applied, and great care was 

 taken to exclude common air as completely as possible. Three 

 trials with gas prepared in this way gave the following results : 



By first trial 1-0409 



By second trial T0410 



By third trial 1-0410 



The mean of these three gives 1-04096 for the specific gravity 

 of deutoxide of azote. This is about , ,'m, th part less than the 

 theoretic specific gravity — a quantity within the limits of unavoid- 

 able error of experiment. Hence 1 consider myself entitled to 

 conclude from the preceding experiments that the true specific 

 gravity of deutoxide of azote is 1-04166. 



Though I have prepared deutoxide of azote many hundred 

 times, I have never been able to obtain it absolutely free from 

 azotic gas. The volume of this gas which it contains sometimes 

 amounts to 10 per cent, and very frequently to 5 per cent. When 

 the quantity is so small as 1 or 2 per cent. I reckon the gas very 

 pure. I do not know the reason of this, unless it be owing to 

 the azote which is always contained in water, and which is 

 evolved when the water is left in contact with nitrous gas. If 



♦ Lavoisier's Elcmenis, p, iCJ. English Traosiiadon. 



