the Congelation of gnickjtker. 
as in molt dilcoveries, much depends upon accident, fo, on this 
occafion, M. braun undertook the experiments for a very dif- 
feient object trom that which prefen ted itfelf in the courfe of 
them, and at the fuggeftion of another perfon. This gentle- 
man was Dr. John ernest zeiher, Profeffor of Mechanics 
in the fame Academy, who having repeated Fahrenheit’s 
expei iments with frigonfic mixtures m Germany, before he 
came to fettle at Petersburg, wifhed to try whether they might 
not be profecuted further in the great natural cold which l'ome- 
times prevails in that city. Illnefs prevented Dr. zeiher from 
carrying his ideas into execution ; he therefore communicated 
them to Profeflor braun, who was already much converfant 
in thermometrical experiments, and engaged him to take up 
the fubjebt of artificial cold whenever the weather fhould be 
favourable for this purpofe. A proper opportunity occurred 
on the 14th of December, 1759, O. S. the thermometer fink- 
ing in the open air fo low as - 34 0 of Fahrenheit’s fcale, 
which we now know to be within a few degrees of the point at 
which mercury freezes. M. braun accordingly prepared a fri- 
gorific mixture with aqua forth and pounded ice, by means 
of which his thermometer was reduced to - 69°, lower, by 
almoft 30 degrees, than it had fallen in any preceding experi- 
ments of this nature. 
Part of the quickfilver had now really congealed, yet fo far 
was M. braun from entertaining a fulpicion of that fort, and 
fo entirely were his views directed to another objedf, that he 
acknowledges he had well nigh defined from all further trials, 
content with having thus much exceeded fuch eminent phi- 
lofophers as Fahrenheit, musschenbroeck, and reaumur. 
Animated, however, by the hope that a hill greater degree 
of cold might be produced, he entered upon the experiment 
anew ; 
