the Congelation of Quickfher. 359 
by plunging them into lome of that metal which lias been 
frozen and is now melting. This was put in practice very iuc- 
cefsfully by Mr. hutchins in his ninth and tenth experi- 
ments. It anfwers to the method of determining the common 
point of congelation upon a thermometer by melting ice, well 
known to be more heady and certain in its temperature than 
freezing water. If, however, the point of mercurial congela- 
tion be not exactly the fame in different portions of the metal, 
it is evident, that no reliance could be placed on fuch an experi- 
ment ; and it can fcarcely be executed but with the greateft 
advantage of natural cold. 
As Dr. guthrie was miflaken in fuppofing he had proved 
that quickfdver did not congeal till it was cooled under - 505% 
his fuggeftion of impurity in the mercury employed by others 
is clearly without foundation. The inftances to which he re- 
fers, when that metal froze bv the natural cold of the air, are 
rendered certain and unexceptionable, from a great variety of 
concomitant circumlhances, confirmed by the moft credible 
tehimony, as fhall prefently be fhewn 
§ 8. This account of mercurial congelation by artificial 
means would remain incomplete, were I not to mention that 
at Hampftead, on the 26th of February laft, the temperature 
of the air being then above + 20% Mr. cavendish, by an in- 
genious artifice of diluting the nitrous acid to a proper degree, 
* Since this was written, Dr. guthrie has fent a more perfed account of his 
experiments. They agree in the main with Mr. hutchins’s, and the difficulties 
which occurred to him may be folved on the fame principles. It feems not impro- 
bable, that the thermometer with which the Dodor ftirred his quickfdver had, by 
fome accident, in the courfe of a long experiment, come into contact with the 
frigorific mixture, and fo been fet freezing. 
Vol. LXXIII. B b b 
funk 
