188 Dr. Thomas Thomson's Observations 



ber deduced from the hypothesis, that air is a compound of 

 20 volumes oxygen and 80 volumes azotic gas ; a hypo- 

 thesis, the truth of which I think I have proved by experi- 

 ment. I consider it then established upon the clearest 

 evidence, that the specific gravity of oxygen gas is 1-^, and 

 that air is a chemical compound of two atoms azote and one 

 atom oxygen. 



From the specific gravity of oxygen gas thus found, that 

 of azotic gas is deducible with absolute certainty, and must 

 be, as I have already shewn, 0-9722. 



Having thus established the specific gravity of oxygen 

 and azotic gases, let us apply this information to the de- 

 termination of the atomic weights of some other substances. 

 1. The atomic weight of sulphur, according to Berze- 

 lius, is 2-01165. Dr. Turner, in his Elements of Chemistry, 

 has adopted this number as he has several others of the 

 atomic numbers given by Berzelius. And he has published 

 in the "Philosophical Transactions" a set of experiments 

 to shew that either my number for sulphur or potassium 

 is wrong. And, when we inspect his book, we perceive by 

 the numbers which he has adopted, that he considers both 

 my numbers as erroneous. 



It has been established by experiments, which I have 

 often verified, that when sulphur is burnt in dry oxygen 

 gas, the bulk of the gas is not altered, but it is converted 

 into sulphurous acid gas. Let us, therefore, determine the 

 specific gravity of sulphurous acid gas. It will enable us to 

 establish the atomic weight of sulphur with much greater 

 accuracy than by any other mode of experimenting. The 

 specific gravity of this gas was determined by Kirwan to be 

 2-265 (On Phlogiston, p. 29.) Davy afterwards took its 

 specific gravity, and found it 2-1930. Now, the mean of 

 these two determinations is 2-229. So that, according to 

 these old determinations, the oxygen gas, when converted 

 into sulphurous acid, is only a very little more than doubled. 

 In the summer of 1820, I made a very careful set of experi- 

 ments on the specific gravity of this gas. It was prepared 

 by boiling sulphurous acid on mercury, and contained no 

 appreciable quantity of foreign matter, being totally ab- 

 sorbed by the peroxide of lead. I took the specific gravity 

 three times in succession, and obtained the following results. 





