48 1 Vapour compofed of [Book VII . 



temperature, compared with that of the furroundirag 

 medium. 



There is one circumftance very much in favour of 

 the opinion, that the emiffion of heat is the chief caufe 

 of evaporation, which is, that every liquor cools when 

 it evaporates, becaufe the portion of the fluid which 

 difappears carries away a quantity of caloric from the 

 liquor, which "becomes latent in the vapour. 



A doubt has arifen among philofophers, whether 

 water, as it exifts in a tranfparent ftate in the atmo- 

 ipherc, is in particles of an uniform denfity, or afTumes 

 the form of hollow veficles. When there is a tendency 

 to rain or fnow, it is known to exift in the ftate of mi- 

 nute drops and icicles j but the folution is then not. 

 perfect, its tranfparency is impaired, and clouds are 

 produced. 



The exittence of hollow fpheres is faid to have been 

 actually obferved by feveral perfonsj but as a micro- 

 fcope of confiderable magnifying power muft be em- 

 ployed, this may have been a fource of error, and may 

 have produced delufive appearances. The fimpleft 

 and moft inftruftive manner of obferving them is to 

 expofe a cup of fome warm aqueous fluid, of a dark 

 colour, as coffee, or water mixed with ink, to the rays 

 of the fun in a fine day, when the air is very calm; 

 a cloud will rife from the fluid to a certain height, 

 and then <lifappear. An attentive eye will foon difco- 

 ver that this cloud confifts of fmall round grains, of a 

 whitifh colour, and detached one from the other. To 

 acquire a more diftinct idea of their form, they may 

 be obferved, as they rife from the furface of the liquor, 

 with a lens of about one inch and an half focus, 

 being careful, however, to keep the lens out of the 

 vapours, .that they may not deprive it of tranfpa- 

 rency. 



Spherical 



