the Congelation of Quickffacr.. 3 6 9 
** were perceived. Next morning only a very few minute air- 
“ bubbles remained in the quickfilver of the thermometer, 
“ which had then rifen to - 44% and not the leaft veftige of 
44 them was to be feen in the barometer.” 
It cannot be doubted, but thefe appearances proceeded from a 
congelation of the mercury in Profeflor gmelin’s inftruments. 
His thermometer fhewed by its defeent that the cold was fuffi- 
cient for this effeft ; and the difappearance of thofe fuppoied 
air-bubbles as the froft abated, demonitrates that they were no- 
thing more than interftices formed by minute portions of con- 
gealed metal refting irregularly upon one another, and which, 
therefore, were gradually obliterated as the iolid bits melting 
down united into one mats. Now this obfervation is of confe- 
\ 
quence, not only as proving that the quicktilver congealed, but 
likewife as pointing out, with great exactnels, the degree or 
cold neceffary for its congelation. For fince, when only a few 
very minute hubbies were lett, the mercury reached up to — 44 
in the thermometer, its freezing point could not be below that 
degree, becauie tome of it continued {fill lolid, but muff be a 
little higher, juft lo much as would aniwer to the expanfion 
produced by the melting of that very fmall proportion of metal 
which remained frozen. 
From Dr. gmelin’s attempt to extricate the fuppofed air by 
means of a wire, it would feem, that the tubes of his ther- 
mometers were open at top. This idea is in fome meafure con- 
firmed by a paflage in the preface to his Flora Sibirica *, where 
he mentions, that upon arriving at, or quitting a place, he 
uied to try whether his thermometer would rife to o in boiling 
water, and, if there appeared any deficiency, corre&edit by the 
addition of frefli quickfilver ; which he would Scarcely have 
* P. lxxiv. 
CtC2 
■done, 
