HEAT. 3 r 



matter with which it is associated changes its state. To take 

 a common example : a pound or given weight of water at 

 172, mixed with an equal weight of water at 32, will acquire 

 a mean temperature, or 102 ; while water at 1/2, mixed with 

 an equal weight of ice at 32, will be reduced to 32. By the 

 theory of latent heat this phenomenon is thus explained : In 

 the first case, that of the mixture of water with water, both 

 the bodies being in the same physical state, no latent heat is 

 rendered sensible, or sensible heat latent ; but in the second, 

 the ice changing its condition from the solid to the liquid state, 

 abstracts from the liquid as much heat as it requires to main- 

 tain it in the liquid state, which it renders latent, or retains 

 associated with itself, as long as it remains liquid, but of 

 which heat no evidence can be afforded by any thermoscopic 

 test. 



I believe this and similar phenomena, where heat is con- 

 nected with a change of state, may be explained and distinctly 

 comprehended without recourse to the conception of latent 

 heat, though it requires some effort of the mind to divest 

 itself of this idea, and to view the phenomena simply in their 

 dynamical relations. To assist us in so viewing them, let us 

 first parallel with purely mechanical actions certain simple 

 effects of heat, where change of state (I mean such change as 

 from the solid to the liquid, or liquid to the gaseous state) is 

 not concerned. Thus, place within a receiver a bladder, and 

 heat the air within to a higher temperature than that without 

 it, the bladder expands ; so, force the air mechanically into it 

 by the air-pump, the bladder expands ; cool the air on the 

 outside, or remove its pressure mechanically by an exhausting 

 pump, the bladder also expands ; conversely, increase the 

 external repellent force, either by heat or mechanical pressure, 

 and the bladder contracts. In the mechanical effects, the 

 force which produced the distension is derived from, and at 

 the expense of, the mechanical power employed, as from 

 muscular force, from gravitation, from the reacting elasticity 

 of springs, or any similar force by which the air-pump may be 

 worked. In the heating effects, the force is derived from the 

 chemical action in the lamp or source of heat employed. 

 Let us next consider the experiment so arranged that the 



