the Point of Mercurial Congelation . 
r • 
J)r . black s Letter referred to above » 
DEAR SIR, Edinburgh, $th 0 &. 1779. 
I HAVE read with great pleafure the experiments made at 
Hudfon’s Bay, upon the congelation of mercury, and obferve 
that the author has fucceeded perfe&ly in effecting it ; but 
could not determine with precifion what degree of cold was 
neceffary to produce it. This, however, does not furprize me, 
as I have always thought it evident, from Pro feffor rrAun’s 
experiments, that this degree of cold cannot be difeovered 
conveniently by congealing the mercury of the thermometer 
itfelf. I fhall not here give my reafons for this opinion ; they 
would lengthen out this letter too much ; I fhall only propofe 
what appears to me the proper manner of making the experi- 
ment, which is as follows : provide a few wide and fhort tubes 
of thin glafs, fealed at one end and open at the other ; the 
widenefs of thefe tubes may be from half to three-quarters of 
an inch, and the length of them about three inches. Put an 
inch or an inch and an half depth of mercury into one of thefe 
tubes, and plunging the bulb of the thermometer into the mer- 
cury, fet the tube with the mercury and the thermometer in it into 
a freezing mixture, which fhould be made for this purpofe in a 
common tumbler or water-glafs ; and, N. B. in making a 
freezing mixture with fnow and fpirit of nitre, the quantity of 
the acid fhould never be fo great as to diffolve the whole of the 
fnow, but only enough to reduce it to the confidence of Pa- 
nada. When the mercury in the wide tube is thus fet in the 
freezing mixture, it (the mercury) muft be ftirred gently and 
*S f 2 frequently 
