(SiSO iECTURE LH. 



in a second bulb, which is not to be exposed to the heat or cold that is to 

 be examined: and the difference between the temperatures of the two bulbs 

 is indicated by the place of a drop of a liquid, moving freely, in the tube 

 M-hich joins them. (Plate XXXIX. Fig. 548 . . 550.) 



The degree of heat, as ascertained by a thermometer, is only to be considered 

 as a relation to the surrounding bodies, iu virtue of which a body supports 

 the equilibrium of temperature when it is in the neighbourhood of bodies 

 equally heated; thus, if a thermometer stands at 60°, both in a vessel of 

 water, and in another of mercury, wc may infer that the water and the 

 mercury may be mixed without an}' change of their temperature: but the 

 absolute quantity of heat, contained in equal weights, or in equal bulks, of any 

 two bodies at the same- temperature, is by no means the same. Thus, in order 

 to raise tlie temperature of a pound of water from 50° to 60°, we need only to 

 add to it another pound of water at 70°, which, while it loses 10° of its own heat, 

 will communicate 10° to the first pound; but the temperature of a pound of 

 mercury at 50° may be raised 10°, by means of the heat imparted to it, by 

 mixing with it one thirtieth part of a pound of water, at the same tem- 

 perature of 70°. Hence we derive the idea of the capacities of different bodies 

 for heat, which was first suggested by Dr. Irvine, the capacity of mercury 

 being only about one thirtieth part as great as that of water. And by similar 

 experiments it has been ascertained, that the capacity of iron is one eighth of 

 that of water, the capacity of silver one twelfth, and that of lead one twenty 

 fourth. But for equal bulks of these different substances, the disproportion 

 is not quite so great; thus, copper contains nearly the same quantity of heat 

 in a given bulk as water; iron, brass, and gold, a little less, silver ■!■ as 

 much, but lead and glass each about one half only. 



It is obvious that if the capacity of a body for heat, in this sense of the word, 

 were suddenly changed, it would immediately becomehotterorcoldtr, according 

 to the nature of the change, adiminution of the capacity producing beat, and an 

 augmentation cold. Such a change of capacity is often a convenient mode of re- 

 presentation for some of the sources of heat and cold : thus, when heat is produc- 

 ed by the condensation ofa vapour, or by the congelation of a liquid, we may ima- 

 gine that the capacity of the substance isdimiuished; and that it overflows, as a 

 vessel would doif its dimensions were contracted. It appears also from directex- 



