J 3 5 JJieut of y<-pour. [Book II. 



Confideration that vapour will afcend in vacuo. The 

 electric fire has alfo been called in to account for this 

 phenomenon j but if the electric fire is no other than 

 common fire, or fome particular modification of it> 

 there is no reafon to believe that the fpontaneous 

 evaporation depends upon any other principle than 

 that which has been already ftated as the caufe of the 

 formation of common vapour from boiling water. 



The fact appears to be, that there exifts fuch an 

 attraction between the particles of caloric and thofe of 

 water, that whenever ji portion of the former in a dif- 

 engaged ftate meets with any of the latter, they mime" 

 diately unite. Hence, when water is heated beyond 

 the temperature of the armofpherc, it naturally yields 

 up a quantity of its fuperfluous fire to reftore the equi- 

 librium, and" this fire always carries with it a quantity 

 of the fluid medium in the form of vapour. When 

 ihcfe vapours firft afcend, they are in an invilible 

 {late ; and they muft be in fome degree condenicd to 

 enable them to reflect the folar rays, ib as to become 

 vifible. This frequently takes place when they reach 

 the higher and colder regions of the atmofphere, or if 

 they happen to meet with cold winds in their progrefs 

 thither : they then appear to us in the form of clouds. 

 A ftill greater degree of cone 1 .?- n&tion renders them too 

 heavy to be ilipported by the atmofphere, and they fall 

 down in the form of rain, fnow, or hail, according to. 

 the circumftances of their difiblution. 



Agreeable to this theory is the common obfrrva- 

 tion, that in very cold weather vapour becomes vifi- 

 ble almoft as foon as it is formed : thus in froil the 

 breath, which is vapour from the lungs, is always vi- 

 fible. M. de Maupertuis faw in Lapland the warm 

 vapour of a room converted into fnow upon opening 

 the door to the external air j and in a crowded afTem- 



My 



