244 Of M U Propagation of Heat 



ing power of water, and resolved to set about making 

 experiments to ascertain the fact. I did not, however, 

 put this resolution into execution till about a month ago, 

 and should perhaps never have done it, had not another 

 unexpected appearance again called my attention to it, 

 and excited afresh all my curiosity. 



In the course of a set of experiments on the commu- 

 nication of Heat, in which I had occasion to use ther- 

 mometers of an uncommon size (their globular bulbs 

 being above four inches in diameter) rilled with various 

 kinds of liquids, having exposed one of them, which 

 was filled with spirits of wine, in as great a heat as it was 

 capable of supporting, I placed it in a window, where the* 

 sun happened to be shining, to cool ; when, casting my 

 eye on its tube, which was quite naked (the divisions of 

 its scale being marked in the glass with a diamond), J 

 observed an appearance which surprised me, and at the 

 same time interested me very much indeed. I saw the 

 whole mass of the liquid in the tube in a most rapid mo- 

 tion, running swiftly in two opposite directions, up and 

 down at the same time. The bulb of the thermometer, 

 which is of copper, had been made two years before I 

 found leisure to begin my experiments, and having been 

 left unfilled, without being closed with a stopple, some 

 fine particles of dust had found their way into it, and 

 these particles, which were intimately mixed with the 

 spirits of wine, on their being illuminated by the sun's 

 beams, became perfectly visible (as the dust in the air 

 of a darkened room is illuminated and rendered visible 

 by the sunbeams which come in through a hole in the 

 window-shutter), and by their motion discovered the vio- 

 lent motions by which the spirits of wine in the tube of 

 the thermometer was agitated. 



