1825.] Specific Qtamty of Hydrdgen Ga&, 137 



being the same. We cannot indeed have vapour at temperature 

 49° and barometer 30 to compare with air at 49° and barometer 

 30 ; but we can have air at 49° and barometer 0*363 to compare 

 with vapour at that temperature and pressure. 



1 consider it to follow as a necessary consequence from Gay 

 Lussac's experiment on vapour, and his theory of volumes, that 

 the following is the true formula for the specific gravity of 

 aqueous vapour. Let the specific gravity of dry air at tempera- 

 ture 60° and barometer 30= 1, and its volume = V ; and let V^ 

 be the volume of air, and p the tension of vapour at any other 

 temperature, then the specific gravity of vapour at that tempera* 

 ture will be 



If in this formula we substitute for V and V the numbers 

 508 and 497, which are the relative volumes at 60° and 49° ; and 

 if for p we substitute 0*363, which is the tension of vapour at 

 49°, we shall find the specific gravity of vapour of maximum 



tension at49° = 1 x ^^ x ^^ x 0*625 = 0*00772, as by the 



former method. 



If in Dr. Thomson's calculations, we substitute the number 

 00772 instead of 0*00533, which he adopts as the specific 

 gravity of vapour at 49°, we shall find that 100 cubic inches of 

 dry hydrogen weigh 2*0537 grains ; and 100 cubic inches of 

 oxygen weigh 33*915 grains at temperature 60° and barometer 

 30 ; and if we admit that in these circumstances 100 cubic inches 

 of dry atmospheric air weigh 30*5 grains, we shall have the spe- 

 cific gravity of hydrogen = 0*0673, the specific gravity of oxy- 

 gen = 1*1 11, and consequently the specific gravity of hydrogen 

 tothe specific gravity of oxygen as 1 to 16*54. 



From this it follows, that if Dr. Thomson's experiment is cor- 

 rect (and of this we can scarcely doubt from the care and atten- 

 tion with which it was performed), it disproves the hypothesis 

 that the specific gravities of all the gases are multiples by inte- 

 ger numbers, of the specific gravity of hydrogen. It is true 

 that 16*54 does not differ from 16 by more than about ^^^ of the 

 whole, and that a very slight change in the number adopted for 

 the specific gravity bi^ hydrogen would account for the differ- 

 ence ; but this merely shows how difficult it is to make any 

 experiment sufficiently accurate to decide on the truth of the 

 hypothesis. 



