3 jo Dr. blAgden’s D'Jlory of 
that which Ihould have rcfulted according to the common laws 
of the diflributiou of heat among bodies ; the other, by ob- 
l'erving how much fader water near the freezing point acquires 
fenfible heat, than an equal quantity of ice melting under fimilar 
circumftanccs. It is obvious, that both thefe methods tend 
not only to prove the fact, but likewife to difeover the quantity 
of heat fo abforbed ; and that the latter alfo, if the operation 
be reverfed, will fhew the quantity of heat evolved, when a 
fluid congeals or becomes folid. In this way Dr. black efti- 
mates the heat in queftion to be equal to 140 degrees upon Fah- 
renheit’s fcale; Profeflor wilcke, by a great variety of ex- 
periments with different proportions of fnow and water, brought 
it out pretty uniformly about 130 ; and Mr. cavendish, as he 
hath lately informed us, finds it amount to 150, and choofes to 
call the procefs a generation of heat or produ&ion of cold *. 
As the method put in practice by Mr. hutchins to fettle the 
freezing point of quickfilver depends entirely upon this gene- 
ration, re-appearance or evolution of heat, by means of which 
the congealing quickfilver is kept at the fame temperature as 
long as any confiderable portion of it remains fluid, I thought 
fome account of fuch an interefling difeovery would not here 
be mifplaced. It now becomes an important object of atten- 
tion, in examining the properties of bodies, to inveftigate the 
quantity of heat produced or loft at their melting and vaporific 
points. So little, however, has hitherto been done in this 
relpedt, even with thofe bodies that are changing from fluid to 
folid every day before our eyes, that it is no wonder we are yet 
unable to determine it in a fubftance which has fo feldom been 
feen in a folid llate. But from the very quick congelation of 
quickfilver when placed in a frigorific mixture, as (hewn by its 
* MelT. lavoisier and de place, from fome late experiments with their new 
apparatus, fix it at 135. 
rapid 
