VOL. LXXIII.] PHILOSOPHICAL TRANSACTIONS. 421 



it more difficult to have communicated a sufficient degree of cold. The diameter 

 of the bulb of the thermometer was rather less than -f of an inch ; that of the 

 swelled part of the cylinder was ■§• ; so that there was no where a much less 

 thickness of quicksilver between the ball and cylinder than -^ of an inch. The 

 bulb of the thermometer was purposely made as small as it conveniently could, 

 in order to leave a sufficient space between it and the cylinder,, without making 

 the swelled part of this larger than necessary ; which would have caused more 

 difficulty in freezing the quicksilver in it. Two of these instruments were sent, 

 for fear of accidents. 



One of the most striking circumstances in the experiments which have been 

 made for freezing mercury, is the excessively low degree to which the thermo- 

 meters sunk, and which, if it had proceeded, as was commonly supposed, from 

 the freezing mixture having actually produced such a degree of cold, would have 

 been really astonishing. The experiments however made at Petersburg afforded 

 the utmost reason to suppose, and Mr. Hutchins's last experiments have put 

 beyond a possibility of doubt, that quicksilver contracts in the act of freezing, 

 or in other words, that it takes up less room in a solid than in a fluid state ; and 

 that the very low degree to which the thermometers sunk was owing to this con- 

 traction, and not to the intensity of the cold produced : for example, in one of 

 Mr. Hutchins's experiments, a mercurial thermometer, placed in the freezing 

 mixture, sunk to 450° below nothing, though the cold of the mixture was 

 never more than — 46 ; so that the quicksilver was contracted not less than 404° 

 by the action of freezing. 



If a glass of water, with a thermometer in it, be exposed to the cold, the 

 thermometer will remain perfectly stationary from the time the water begins to 

 freeze, till it is entirely congealed, and will then begin to sink again. In like 

 manner, when a thermometer is dipped into melted tin or lead, it remains per- 

 fectly stationary, from the time the metal begins to harden round the edges of 

 the pot till it is all become solid, when it again begins to descend ; and there was 

 no reason to doubt that the same thing would obtain in quicksilver. 



From what has been just said it was concluded, that if this apparatus was put 

 into a freezing mixture of a sufficient coldness, the thermometer would immedi- 

 ately sink till the quicksilver in the cylinder began to freeze, and would then 

 continue stationary, supposing the mixture still to keep cold enough, till it 

 was entirely congealed. This stationary height of the thermometer is the point 

 at which mercury freezes ; though, in order to make the experiment convincing, 

 it was necessary to continue the process till so much of the quicksilver in the 

 cylinder was frozen as to put the fact out of doubt. 



If the experiment had been tried with no further precautions, it is apprehended 

 that considerable difficulties would have occurred, from want of knowing whe- 



