the Congelation of Quickfilver. 3 5 1 
rapid defcent in the thermometer, and from its rcadinefs to 
melt again upon an abatement of the cold, apparent in all the 
experiments, and particularly noticed by M. braun, there is 
lealon to believe, that the Quantity of heat employed in giving 
it fluidity is not very confiderable. When water, which has 
been cooled below + 32 0 , begins to freeze, a certain part of it, 
proportioned to the degree of cooling, (hoots at once into ice ; 
that is, ice continues to be formed till fo much heat be evolved 
as is requifite to bring the whole up again to 4- 32 0 . Now I 
am inclined to fufpeCt, that in feveral of Mr. Hutchins’s ex- 
periments the firftjump of the quickfilver down from a little 
below the point of mercurial congelation, depended on a firm- 
lar principle of the fudden freezing of fuch a proportion of the 
mercury as correfponded to the number of degrees it had been 
cooled below that point ; hence, if the degree to which it bore 
to be cooled before it began to congeal, and the contraction it 
buffers in congealing, were both known, its quantity of latent 
heat, to fpeak in Dr. black’s language, might readily be 
found. From a rude and vague computation of this fort, I 
am led to believe, it is not half that of water ; and if lb, 
quickfilver feems to differ much in this rebpeCt from other me- 
tals ; for tin is laid, from Mr. irwin’s experiments, to re- 
quire, in order to melt, a quantity of heat which, if fet loofe 
and rendered fenfible, would raife the thermometer 500 degrees. 
Belides the inftruments contrived particularly to try the 
freezing point of quickfilver, two fpirit-thermometers alfo were 
bent out to Hudlbn’s Bay, principally with a view to borne col- 
lateral circumltances of the experiment. My intention in 
recommending them was to difeover what degree of cold the 
freezing mixture produced ; and to obtain a more exaCt com- 
panion of the relative contractions of mercury and alcohol, by 
Vol. LXXIIJ. A a a marking 
