242 STATE OF VAPOUR SUBSISTING IN THE ATMOSPHERE. 



V. 



Of the State of Vapour fubfijling in the Atmofphere. By Rich akd 



KirwaJ<, Esq. LLD. F. R. S. and P. R. I. A*. 



{Concluded froin- Page 215.) 



R. SAUSSURE, Hygrometer, p. 284-, has given us the 

 fpecific gravity, not indeed of pare vapour, but of vapour dif- 

 folvedin air, with more precifion, for he tells us, J. That a cu- 

 bic foot of perfectly dry air has its volume augmented by -^ 

 when faturated with ten grains of moifture 3t about 65° Fahr. of 

 heat, and barometer 28,77 inches (Englifh). 

 Specific gravity 2. That a cubic foot of pure or perfectly dry air of that den- 

 moil^air.* 11 ^J anc * al tnat temperature weighs 751 grains (French), and 

 after diflblving 10 grains of moifture, by which it is dilated 7 X T , 

 this new volume weighs 751 -f- 10 = 761 grains; but a cubic 

 foot of pure air augmented by an acceffion of I of its bulk of 

 pure air would weigh 751 -f- -^ 751 = 765 grains, that is 14 

 grains more. Hence he infers that in this cafe the fpecific gra- 

 vity of the diflblved moifture is to that of dry air as 10 to 14, 

 for -jJy of a cubic foot in the one cafe weighs 10 grains, and in 

 the other 1 4 grains nearly. 

 Sufpefted errors But I ftrongly fufpeel that the original experiment, on which 



in the original ^ Ca ] €ll i a ti n is founded, is erroneous, chiefly by reafon of the 

 experiment* J J 



ftrong adherence of moifture to cold glafs, as will hereafter be 



feen in treating of dew. From Schmidt's experiments, it may 

 be inferred, that the fpecific gravity of vapour diflblved in air 

 at this temperature is much lower with refpeft to that of pure 

 air than Sauflure has ftated, for he tells us that about 1066 

 meafures of dry air in temperature 65° would, if faturated with 

 moifture at that temperature, occupy the fpace of about 1 100 

 meafures, and confequently receive an augmentation amount- 

 ing to about y'L of their bulk ; now, transferring this ratio to 

 the cubic toot in Sauflure's experiment, it appears that T L of a 

 cubic foot thus added to the cubic foot of dry air weighs 10 

 grains ; but a cubic foot of dry air, augmented by an acceffion 

 of y'-y of flmilarair, would weigh 751 -f- 23,46 grains, which 

 approaches nearly to Mr. Watt's ratio, therefore the fpecific 

 gravity of vapour diflblved in air at this temperature is to that 

 of perfectly dry air as 10 to 23,5 nearly. It ftiould however 



be 



