THE MEASURES AND THE NATURE OF HEAT. 499 



of the instrument. The elasticity of the air contained in the bulb is here 

 counteracted, not by the pressure of a column of mercury, but by the elas- 

 ticity of another portion of air 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 which joins them. (Plate XXXIX. Fig. 548... 

 550.) 



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

 sidered as a relation to the surrounding bodies, in 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, we may infer that the water 

 and the mercury may be mixed without any 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 the temperature of a pound of w r ater 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 temperature 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 ascer- 

 tained, 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 become hotter or 

 colder, according to the nature of the change, a diminution of the capacity 

 producing heat, and an augmentation cold. Such a change of capacity is 

 often a convenient mode of representation for some of the sources of heat 

 and cold : thus, when heat is produced by the condensation of a vapour, 

 or by the congelation of a liquid, we may imagine that the capacity of the 

 substance is diminished ; and that it overflows, as a vessel would do if its 

 dimensions were contracted. It appears also from direct experiments, in 

 some such cases, that the capacity of the same substance is actually greater 

 in a liquid than in a solid state, and in a state of vapour, than in either ; 

 and both Dr. Irvine and Dr. Crawford t have attempted to deduce, from a 

 comparison of the proportional capacities of water and ice, with the quan- 

 tity of heat extricated during congelation, a measure of the whole heat 

 which is contained in these substances, and an estimation of the place 

 which the absolute privation of heat, or the natural zero, ought to occupy 

 " in the scale of the thermometer. Thus, when a pound of ice, at 32, is 

 * Chemical Essays. f On Animal Heat, &c. 2nd edit. 1788. 



2 K 2 



