the Congelation oj Quickfilver, 355 
“ phenomenon that always precedes congelation, as I have 
found in my experiments; nor had there as yet been formed 
m the mlide of the tube containing the mercury to be frozen 
(and the thermometer to determine the point of congelation 
“ with which I birred it) an incruftation of the metal, another 
“ indication of approaching congelation, which ever begins on 
“ the fide of the tube, and gradually increafes till it has 
“ reached the center, and a folid cylinder is produced. 
“ From this you may form a judgement of the impurity of 
“ the mercury which fome pretend to have feen congealed with 
“ natural cold ; for here 1 50° of reaumur was not found a fuf- 
“ hcient degree of cold to freeze it, andfurelyno fucli abfence 
“ of heat, or any thing near it, has ever been, or ever could 
“ be, obferved on the face of the habitable globe. 
I fhall only add, that my experiments were conduced on 
** the plan of my learned friend Dr. black, and fpiritus nitri 
“ fumans Glauberi with fnow, were employed to produce an 
44 artificial cold, while the thermometer of reaumur flood at 
44 20 0 below o in the open air” [that is, - 13 0 of Fahren- 
heit’s fcale.J 
Though the confequences here deduced by Dr. guthrie 
from his experiment are undoubtedly erroneous, as appears from 
a fufficient number of other fads, yet it is not at all furprizing 
that they fhould have feemed to him juft ; for the error arifes from 
a circumflance, which could not be forefeen with certainty, and 
occurred in feveral of Mr. hutchins’s experiments as well as 
in Dr. guthrie’s. To underhand this, it mult be confidered, 
that when we attempt to afcertain the freezing point of water, 
by keeping a thermometer immerfed in it while it is changing 
into ice, the inftrument employed for this purpofe is not made 
of water, but of a different fluid, not fubjed to be peculiarly 
affeded 
