thc'ir Prodiuiion, Suspcnst07i, and D^slntcl'iGn. 351 



3dly, The vapour thus thrown into the atmosphere is dif- 

 fusible therein by xU own elasticity, which suffices for its 

 ascent to any height in a perfect calm. Yet, iia in tiiis case 

 the inertia of the particles of air considerably resists its dif- 

 fusion, so in tile opposite one of a brisk current, the vapour, 

 by the same rule, must in some measure be drawn aloniv with 

 the mass into which it enters. 



4thly, The qiumtity ot" vapour which, under equal pres- 

 sure, can subsist in a given mass of air, will be greater as 

 the common temperature is higher, and vice versa *. 



Aqueous vapour is the only gas contained in the atmo- 

 sphere which is subject to very sensible variations in quan- 

 tity. These variations arise from its attraction for caloi ic 

 being inferior to that of all the others. Hence v.hcn a cold 

 body, such as the glass of water in the experiment above 

 quoted, is presented to the atmosphere, the other gases, 

 composing the latter, will only be cooled by it (and that at 

 all known temperatures) ; but the \apour, after being more 

 or less cooled, will begin to be decomposed, its caloric en- 

 tering the body while the water is left on the surface. 



The formation of cloud is in all cases llie remaie conse- 

 quence of a decomposition thus eflectcd, except that the ca- 

 loric escapes, not into a solid or liquid, but into the sur- 

 rounding gases. 



Of the Formation of Deiv. 



Dew is the immediate result of this decomposition. The 

 particles of water constituting it are, singly, invisible, on ac- 

 count of their extreme minuteness. The approach of dew 

 is, nevertheless, discoverable by a dark ha/,y appearance, 

 verging from purple to faint red, extending from the horizon 

 to a small distance upwards, and most conspicuous over val- 

 leys and large pieces of water. 



The theory of dew seems to be simply this : — Durine; the 

 heat of the day a great quantity of vapour is thrown into the 

 atmosphere from the surface of the earth and waters, ^^■l!en 



*"Thc aiineous vapour atmosphere is variable in quantity according to 

 tempeiiiture : in the torrid zone its pressure on tlu. surface of the earth is 

 cqudlto the force of "6, and from tliat to one in<:h of mercury. In tluse 

 parts it rarely amounts to the pressure of -6, but 1 have frctnicntly observed 



it above half an inch in summer: _in winter it is torac'.i.."-- "• • :••.; f; re (.'f 



no more force ih.m -i of an inch of mercurv, rr e'.en half a t;;mh, in ihis 

 latitude, and cor.-^cqucntly :i:v.th l:ss where ihc cold is more slvcic. Tli.'s 

 want of etiuilibriii:n in the aqueous vapour attncrphcre is a principal cause 

 of that constant i .und:!tion of it into the temperaie and fri-'id ?oncs, whepc 

 it becomes in part c^mdenscd in its progress by ihe cold, J;l:e the vapour 

 of distii.ution in the worm of a rtfiigcuiory, ami supplai tlie caith witli 

 rain and de\v." See :ae Essavs above quoted. 



5 ' the 



