2$6 Dr. blag den’s Hi/lory of 
affeCted by that particular degree of cold. In order, therefore* 
to render the experiment with quickfilver perfectly analogous, 
it would be neceffary not to make ufe of a mercurial thermo* 
meter ; but to fubbitute fuch a one as is capable of fubaining 
a greater intenfity of cold. For other wife, if it ihould hap- 
pen, from any circumbance, that the quickfilver in the ther- 
mometer fhould begin to freeze before that in which it is 
plunged, the whole experiment mub evidently be fruitlefs, as 
the former would fink, perhaps, many hundreds of degrees in 
the inbrument, by its own contraction in becoming folid, 
while the furrounding mercury ftill retained its fluidity. Now 
this was precifely the cafe in Dr. Guthrie’s experiment; the 
thermometer, with which he birred his quicklilver, congealed, 
it would feem from the great defcent, almob entirely, though 
he could not perceive in the quickfilver fo agitated the leafl ap- 
pearance of change to a folid bate. Thus, likewife, in feveral 
of the experiments at Hudfon’s Bay, the mercury in the en- 
clofed thermometer was found to freeze before that in the cy- 
linder. Hence it is manifeb, that the continuance of fluidity 
in a quantity of quickfilver does not fecure a thermometer of 
that metal immerfed in it from freezing. 
The caufe of this phenomenon is extremely uncertain. Pof- 
fibly the point of congelation may not be exactly the fame in 
all quickfilver under all circumbances. Foreign admixtures 
may occafion a difference in this refpeCt ; and it does not follow, 
that the effeCt of fuch, in certain proportions, mub neceflarily 
be to make the mercury congeal fooner, lince, in the cafe of the 
fufible metal, the melting point of till is brought lower by the 
addition of two metallic fubbances, both of which feparately 
require a bronger heat than it for their fufion. 
But as quickfilver bears to be cooled fome degrees below its 
freezing point, before it begins to form folid crybals, the phe- 
7 nomenon 
