124 Review of Dr. Thomson 



of aqueous vapour exists in moist air, as would be found, at the 

 same temperature, in a vacuum of the same capacity. Thus, 100 

 cubic inches of moist air, at 60°, will contain 100 cubic inches of 

 aqueous vapour at 60° ; which, by Saussure's experiments, weigh 

 0.35 grain. Dividing this number by 30.5, the weight of 100 

 cubic inches of dry air, the quotient 0.0114 will be the specific 

 gravity of vapour at 60°, referred to air as unity. 



According to M. Gay Lussac, the specific gravity of steam at 

 212° is to air at 212°, under the barometer pressure of 30 inches, 

 as 0.625 to 1.000. But vapours out of contact of liquids, and 

 air, follow the same rate of expansion or contraction, by change 

 of temperature. This ratio of 0.625 to 1.000, holds therefore at 

 all temperatures ; and we have therefore merely to estimate the 

 diminution of the quantity of vapour, due to its diminished ten- 

 sion. Now from the latest table of elastic forces of steam, that 

 published in the Philosophical Transactions for 1818, we learn 

 that the tension at 60° is represented by 0.516 of an inch of 

 mercury ; and consequently, that the weight of vapour is dimi- 

 nished in the ratio of 0.516 to 30 inches. But °' 625 X ?' 516 = 



30 



0.01075 ; which is the specific gravity of aqueous vapour at 60o 

 Fahr. ; and 100 cubic inches of it will weigh 0.327875 == 30.5 

 X 0.01075, or pretty nearly one-third of a grain. 



Gay Lussac has also shewn that aqueous vapour, at any tem- 

 perature and pressure, consists of a volume of hydrogen, + half 

 a volume of oxygen, constituting together one volume. Now a 

 volume of hydrogen, therm. 60°, and barometric pressure 0.516, 



• ^^/mi«o 0.0694 X 0.516 , , ir , r 



is 0.001193 sr ; and halt a volume of oxygen 



. ., . a -*m»*i>* 0.5555x0.516 riM. - 



m the same state is 0.009555 = . The sum of 



30 



these two quotients is 0.01075 ; precisely as deduced above, in 

 a different manner. 



One hundred cubic inches of air, barom. 30, standing 

 over water at 60° Fahr., consist of 100 cubic inches of dry air 

 supporting a barometric pressure of 29.484 inches of mercury, 

 and 100 cubic inches of vapour, sustaining a pressure of 0.516 of 

 an inch of mercury. But 100 cubic inches of dry air, under that 

 pressure, weigh 29.9754 grains, and 100 cubic inches of vapour 

 under its pressure weigh 0.3278 grains. The sum 30.3032 is the 

 weight of the hundred cubic inches of moist air, sustaining the 

 total pressure of 30 inches of mercury. 



In like manner, 100 cubic inches of oxygen gas standing over 

 water, at 60° Fahr., consist of 100 cubic inches, sustaining a 

 pressure of 29.484 inches of mercury, and 100 cubic inches of 



