C 43 ] 
IV. Frigorific Experiments on the mechaiwal ex- 
pansion of Air, explaining the Caufe of the great Degree of 
Cold on the Summits of high Mountains , the fudden Condenja - 
tion oj aerial Vapour , and of the perpetual Mutability of 
atmofpheric Heat . J 3 y Erafmus Darwin, M. D. F. R. S. ; 
communicated by the Right Honourable Charles Greville, 
F. R. S. 
Read December 13, 1787. 
H 
AVING often revolved in my mind the great degree of 
cold producible by the well known experiments on 
evaporation ; in which, by the expanfion of a few drops of 
ether into vapour, a thermometer may be funk much below 
the freezing point ; and recolle&ing at the fame time the 
great quantity of heat which is neceflary to evaporate or con- 
vert into fleam a few ounces of boiling water ; I was led to 
fufpeft, that elaftic fluids, when they were mechanically ex- 
panded, would attrafl or abforb heat from the bodies in their 
vicinity ; and that, when they were mechanically condenfed, 
the fluid matter of heat would be preflfed out of them, and 
diffufed among the adjacent bodies. 
As this principle might poflibly be extended to elaftic folid 
bodies, as well as to fluid ones, and explain the caufe of the 
heat occafioned bv percuflion or fridlion, and by fome chemi- 
cal combinations, as well as the perpetual mutability of it in 
G 2 the 
