6 
THE VOYAGE OF THE VEGA. 
[chap. 
up in the thermometer case. When mercury freezes in a 
common thermometer, it contracts so much that the column of 
mercury suddenly sinks in the tube, or if it is short, goes wholly 
into the ball. The position of the column is therefore no 
measure of the actual degree of cold when the freezing takes 
place. The reading of —89°, or even of —150°, which at a 
time when it was not yet known that mercury could at a low 
temperature assume the solid form, was made on a mercurial 
thermometer in the north of Sweden,^ and which at the time 
occasioned various discussions and doubts as to the trustworthi¬ 
ness of the observer, was certainly quite correct, and may he 
repeated at any time by cooling mercury under its freezing-point 
in a thermometer of sufficient length divided into degrees under 
0°. The freezing of mercury ° takes place from below upwards, 
the frozen metal as being heavier sinking down in that portion 
which is still fluid. If when it is half frozen the fluid be poured 
away from the frozen portion, we obtain groups of crystals, 
composed of small octohedrons, grouped together by the edges 
of the cube. None of our mercurial thermometers suffered any 
damage, nor was there any alteration of the position of the 
freezing-point in them from the mercury having frozen in them 
and again become fluid. 
1 And. Hellant, Anmdrhiingar om enhelt ovanlig hold i Tome {Remarks 
on a Quite Unusual Cold in Tome), Vet.^akad. Hand!. 1759, p. 314, and 
17G0, p. 312. In the latter paper Hellant himself shows that the colnnin 
of mercury in a strongly cooled thermometer for a few moments sinks 
farther when the ball is rapidly heated. This is caused by the expansion 
of the glass when it is warmed before the heat has had time to com¬ 
municate itself to the quicksilver in the ball, and therefore of course can 
happen only at a temperature above the freezing-point of mercury. 
2 That mercury solidifies in cold was discovered by some academicians 
in St. Petersburg on the 25th December, 1759, and caused at the time a 
great sensation, because by this discovery various erroneous ideas were 
rooted out which the chemists had inherited from the alchemists, and 
which were based on the supposed property of mercury of being at the 
same time a metal and a fluid. 
