VOL. LXXIII.] PHILOSOPHICAL TRANSACTIONS. 425 



manner above described. In the first experiment the ivory thermometer, in- 

 closed in the cylinder, sunk to — 40°, where it remained stationary for about 

 half an hour, though the wooden thermometer, placed in the same mixture, 

 kept sinking almost all the while. At the end of that time the apparatus was 

 taken out of the mixture to be examined, and the quicksilver in the cylinder was 

 found frozen. It seems evident therefore, that the true point at which mercury 

 freezes is 40° below nothing on the thermometer f, which was that made use of 

 in the experiment. It cannot be lower than that, for if it was, the thermometer 

 could not have remained so long stationary at that point, while surrounded with 

 freezing quicksilver ; and it cannot be higher, as the thermometer could not sink 

 below the freezing point, while much of the quicksilver, with which it was sur- 

 rounded, remained unfrozen. 



In the 2d exper., tried with the same apparatus, the ivory thermometer quickly 

 sunk to — 43°; but, in about half a minute, rose to — 40°, where it remained 

 stationary for upwards of 17 m . It appears therefore, that in this experiment the 

 quicksilver was cooled 3° below the freezing point, without losing its fluidity ; it 

 then began to freeze, and the inclosed thermometer immediately rose to — 40° : 

 so that this experiment, besides confirming the former, shows that quicksilver is 

 capable of being cooled a little below the freezing point without freezing; and 

 and that it suddenly rises up to it as soon as it begins to lose its fluidity. In this 

 experiment the cold was carried far enough to freeze the quicksilver in the ivory 

 thermometer, which was not the case in the former : for after it had remained 

 I7 m stationary at — 40°, it began to sink again, and in about a minute sank to 

 — 44°4- ; it then sank instantaneously to — Q2°, and soon after remained fixed 

 for an hour and a quarter at 05° ; being then left without examination for three- 

 quarters of an hour, the mercury was found to have sunk into the ball, the spirit 

 thermometer showing at that time that the mixture was rather above the point of 

 freezing, whereas before it had been below it. It appears therefore, that the 

 quicksilver in the thermometer, after having descended to — 44°4-, froze in 

 the tube, and stuck there ; but, being by some means loosened, sunk instantly 

 to — 92°, and again stuck tight at Q5°, till at last the mixture rising above the 

 freezing point, the quicksilver in the tube melted, and sank into the ball, to 

 supply the vacuum formed there by the frozen quicksilver. A similar accident of 

 the quicksilver freezing in the tube of the thermometer, and sticking there, and 

 then melting and sinking into the ball as the weather got warmer, has been found 

 by Dr. Blagden to have happened to several gentlemen whose thermometers froze 

 by the natural cold of the atmosphere, and with reason caused much perplexity 

 to some of them. In this experiment the apparatus was not taken out to be 

 examined till the ivory thermometer had sunk to — p5° ; it was then found to be 

 frozen solid. 



vol. xv. 3 I 



