422 PHILOSOPHICAL TRANSACTIONS. [ANNO 1783. 



ther the cold of the mixture was sufficiently great, and when a sufficient quantity 

 of the quicksilver was frozen ; for, in the first place, there would be no judging 

 when a sufficient quantity was frozen without taking out the apparatus now and 

 then to examine it, which could not be done without a loss of cold ; and, what 

 is still worse, if before the experiment was completed the cold of the mixture was 

 so much abated as to become less than that of congealing mercury, the frozen 

 quicksilver would begin to melt, and the operator would have no way of detect- 

 ing it, but by finding that great part of his labour was undone. For this reason 

 two other mercurial thermometers were sent, called a and b by Mr. Hutchins, 

 the scales of which were of wood, for which reason I shall call them, for short- 

 ness, the wooden thermometers, as I shall call the two others the ivory ones, 

 their scales being of that material ; they were graduated to about 600° below 

 nothing, and their balls were nearly equal in diameter to the swelled part of the 

 cylinders, in order that the quicksilver in both should cool equally fast; and it 

 was recommended to Mr. Hutchins to put one of these into the freezing mix- 

 ture along with the apparatus : for then, if the cold of the mixture was suffi- 

 cient, both thermometers would sink fast till the quicksilver in the cylinder began 

 to freeze, when the ivory thermometer would become stationary, but the wooden 

 one would still continue to sink, on account of the contraction of the quick- 

 silver in its ball by freezing ; but if this last thermometer, after having continued 

 to sink for some time after the ivory one had become stationary, ceased at last 

 to descend, it would show that the mixture was no longer cold enough to freeze 

 mercury ; for as long as that was the case, the wooden thermometer would con- 

 tinue to descend by the freezing of fresh portions of quicksilver in its ball, but 

 would cease to do so as soon as the cold was at all less than that. As I was 

 afraid, however, that the quicksilver might possibly freeze and stick tight in the 

 tube of this thermometer, and prevent its sinking, which would make the cold 

 of the mixture appear too small when in reality it was not, one of these thermo- 

 meters, instead of having a vacuum above the quicksilver as usual, was made 

 with a bulb at top filled with air, that the pressure might serve to force down 

 the quicksilver. If the degree of cold at which mercury freezes had been known, 

 a spirit thermometer would have answered better ; but that was the point to be 

 determined. 



As it appeared, from Mr. Hutchins' s table of comparison, that these ther- 

 mometers did not agree well together, they were all examined after they came 

 back, except the ivory thermometer f, which was broken before it arrived. 

 This loss, however, is of little consequence, as it appeared from his experiments 

 that F and g agreed well together. The boiling and freezing points were first 

 examined, when the divisions on the scale answering to them were found to be 

 as follows 



