[ 148 J 
coldnefs of the air ; for water is gradually con- 
denfed by cold till the moment it freezes, and fince 
it evaporates even when frozen into hard ice, if 
muft alfo evaporate in all the lelTer degrees of cold. 
Now Mr. Boyle having counterpoifed a piece of ice 
in a fcale, hung it out in afrofty night, and found next 
morning that it had loft confiderably of its weight by 
evaporation. Who (fays he) would have thought 
that fo extremely hard and cold a body would evapo- 
rate fo faft in the clear air of a freezing night?’' And 
fince- that time others have obferved the fame thing; 
which fadt feems to be an unanfwerable objedfion to all 
the accounts in which rarefadtion by heat is.made to be 
the chief, if not the only caufe of evaporation ; and, 
therefore, we muft have recourfe to fome other 
principle to affift us in accounting for this plneno- 
menon. 
As the author of nature does not employ in his 
works^ a greater variety of caufes than is abfolutely 
neceftary; it is the bufinefs of natural philofophy to 
reduce as many phaenomena as may be to fome ge- 
neral well known caufe ; and this is to be done by 
comparing the phasnbmena together in their feveral 
circumftances, in which, if they are found to agree, 
they are then to be confidered as effedls of the lame 
kind, and aferibed to the fame caufe. By which means 
the caufes, whofe exiftence is already proved, will 
be rendered more general, and our knowledge more 
extenlive. Now as the fufpenlion of the particles 
of water in air, of fait in the waters of the ocean, 
and of other heavy bodies in the fluids that diflblve 
them, feem to be pha:nomena of the fame kind, 
we might reafonably fuppofe that they arife from 
the 
