FROM THE HIGHER ATMOSPHERE. 469 
quent buoyancy, and spread over the surface. Ina short while, 
therefore, the heat is distributed through the whole liquid mass, 
in successive strata, from the bottom to the top. But liquids 
also conduct heat by a slow process in every direction, like 
solids. This protracted diffusion of influence, which is’ exert- 
ed in restoring the equality of temperature through the body 
of the liquid, succeeds to the operation of the principle of 
buoyancy. While the surface becomes again colder, the bottom 
grows gradually warmer ; the heat working downwards, by a 
continual transfer, from stratum to stratum, of the stagnant 
Flak sit v" 
' The diffusion of heat, then, depends chiefly on the expan- 
sion and internal mobility of the liquid medium. Thus, alco- 
hol rapidly diffuses heat, while the viscid oils, especially at low 
temperatures, clog its motions. The circulation is in general 
quicker when the liquid is very warm, its expansibility, and’ its 
aptitude for internal: migrations being now increased. In-ap- 
proaching the boiling point, water expands largely by equal’ac- 
‘cessions of heat ; but near congelation, its-expansions or contrac- 
tions are extremely small. The conducting power of water at 
a very low temperature, is hence nearly the same as if it were 
a solid, or remained motionless; the Same, in short, as if by 
the addition of isinglass, it were changed into a thin jelly. 
Such are the conducting powers which a liquid substance 
combines when at rest, or left merely to the play of its internal 
motions. But if made to flow in a stream, it will evidently, in 
consequence of the frequent renewal of contact, abstract heat 
more quickly from every warm solid body which is immersed in it. 
Thus, I find, that, in ordinary circumstances, water, advancing 
at the slow rate of a mile in three hours, will yet conduct away 
heat twice as fast as when quite stagnant. Consequently a cur- 
rent 
