On the Constitution of mixed Gases, Sc. 547 
cause it preserves its elasticity in all observable 
temperatures. 
The aqueous vapour atmosphere is variable in 
quantity according to temperature; in the torrid 
zone its pressure on the surface of the earth is 
equal to the force of ,6 and from that to one 
inch of mercury. In these parts it rarely 
amounts to a pressuse of ,6, but I have fre. 
quently observed it above half an inch in sum. 
mer; in winter it is sometimes so low as to be 
of no more force than ,1 of an inch of mercury, 
or even half a tenth, in this latitude, and conse- 
quently niuch less where the cold is more se- 
vere.* This want of equilibrium in the aqueous 
vapour atmosphere is a principal cause of that 
constant inundation of it into the temperate and - 
frigid zones, where it be comes in part condensed 
in its progress by the cold, like the vapour of 
distillation in the worm of a refrigeratory, and 
supplies the earth with rain and dew. 
The carbonic acid atmosphere has not perhaps 
been accurately ascertained in quantity; it is 
found every where in a small proportion, not 
- being condensible into a liquid by the usual de- 
gree of cold; its pressure may probably amount 
to half an inch of mercury. 
* The means of ascertaining its quantity or pressure 
will be given hereafter, 
