186 Dr. Thomas Thomsons Observations 



The next experimenter is M. T. de Saussure, who, in his 

 " Observations on the Combustion of different kinds of Char- 

 coal, and on Hydrogen Gas" published in the year 1809. 

 (Annates de Chimie, lxxi, 254,) gives a determination of the 

 specific gravity of oxygen gas loaded with humidity at 54°|. 

 According to his statement a cubic decilitre of oxygen gas, 

 at the temperature of 54°£, and when the barometer stands 

 at 29-834 inches, weighs 1-3552 French grains. It follows 

 from this, that at the temperature of 60°, and under the 

 same barometrical pressure 100 cubic inches of oxygen gas 

 weigh 35-2018 grains troy. Now, if 100 cubic inches of 

 air at 60°, and under the mean pressure of the atmosphere, 

 weigh 31-0117 grains, as results from the experiments of 

 Dr. Prout, the specific gravity of oxygen gas, as determined 

 by De Saussure, is 1-13521. Notwithstanding the great care 

 which De Saussure bestowed on this experiment, there can 

 be no doubt, that his determination is too high. For, if 

 air be composed of 21 volumes of oxygen and 79 volumes 

 of azotic gas, and the specific gravity of common air 1, 

 then the specific gravity of azotic gas deduced from Saus- 

 sure's number for oxygen would be 0*96405, which is cer- 

 tainly below the truth. If air be a compound of 20 vo- 

 lumes oxygen and 80 azotic gas, the specific gravity of 

 azotic gas would be only 0*96369 which is still lower. 



In the year 1820, I devoted almost the whole of the sum- 

 mer to the determination of the specific gravity of gases. 

 I got an apparatus constructed for the purpose, and was at 

 uncommon pains, both in preparing the gases, and in ob- 

 taining them in a state of as great purity as possible. The 

 oxygen gas was prepared from chlorate of potash, and con- 

 tained no sensible admixture of azotic gas. My mode of 

 proceeding was to weigh a flask filled with dry air ; to ex- 

 haust the flask, and weigh it in that state. This gave the 

 weight of air removed from the flask by the air pump. The 

 flask was then filled with the oxygen, taking care to allow 

 it to remain in contact with the oxygen in the gas holder, 

 till it had acquired the temperature of the room. The flask 

 was then weighed. The increase of weight gave the weight 

 of a volume of oxygen gas equal to that of the air with- 

 drawn from the flask by the air pump. I had only to divide 

 the weight of the oxygen by that of the air, the quotient gave 



