C 177 ] 
Its particles repel each other according to its different 
degrees, we may reafonablyfuppore that thefe particles, 
from their being in contadl with the bottom of the 
veffcl, may, at length, acquire fuch a degree of heat 
as will give them a repelling force fufficient to over- 
come the preffure they fuftain, and expand them 
fuddenly into thofe large bubbles that afcend through 
the water when it boils violently. I have lately 
made fome obfervations and experiments, which 
feem very much to favour this opinion. Thefe 
bubbles, which afcend from the bottom, I obferved 
always grow lefs before they reach the furface, and 
thofe fmall bubbles, which adhere to the bottom, 
for fome time before they afcend, often difappear 
intirely before they reach the furface ; which fhews, 
that when the matter they contain, or any part of 
it, lofes the heat it had at firlf, it is turned into wa- 
ter. When water, that has juft boiled, is poured into 
a glafs, and fet under the receiver of an air pump, and 
the air is almoft drawn out, the water appears to 
boil more violently than it does on the tire; and 
the bubbles do not all rife from the bottom, but 
break out from other parts of the water, efpecially 
towards the middle, where we may fuppofe it hotteft. 
In this cafe no fubtile fluid can be fuppofed to rife 
through the bottom of the veffel ; but the heat, 
which the water retains, will give its particles an 
elaftic force fufficient to overcome the preffure of 
what little air remains in the receiver, and expand 
them into bubbles; and that thefe bubbles are 
corapofed of fteam appears plainly from this expe- 
riment, for as foon as they begin to afcend, the re- 
ceiver is filled with fteam, which being condenfed 
V-OL. LV. A a by 
