62 CORRELATION OF PHYSICAL FORCES. 



duces other effects in the fluid than heat, produces only heat, 

 or is assisted by molecular forces in generating heat, and 

 according to the quantity of heat, greater than, equal to, or 

 less than that held by the fluid in its primitive condition, 

 which it must hold to keep itself at the same temperature 

 when compressed. The a priori assumption of equivalence 

 for the case of air, without some special reason from theory or 

 experiment, is not less unwarrantable than for the case of any 

 fluid whatever subjected to compression. Yet it may be 

 demonstrated that water below its temperature of maximum 

 density (39* I Fahrenheit), instead of evolving any heat at all 

 when compressed actually absorbs heat, and at higher tem- 

 peratures evolves heat in greater or less, but probably always 

 very small, proportion to the equivalent of the work spent ; 

 while air, as will be shown presently, evolves always, at least 

 when kept at any temperature between o and 100 cent, 

 somewhat more heat than the work spent in compressing it 

 could alone create.' 



These remarks show some of the difficulties, among many, 

 which present themselves in the way of ascertaining a me- 

 chanical equivalent of heat ; the most striking is perhaps the 

 expansion of water, and some other substances, at a certain 

 temperature by cold, or, if the expression be preferred, by the 

 absorption of heat by a cold body. 



Thus, assume a cylinder of water at its point of maximum 

 density, 39, which \vater supports a piston with a weight on 

 it. Now let a substance colder than the water, say ice at 25, 

 be brought into contact with the cylinder, the water will 

 expand by the cooling effect of the ice, and produce media 

 nical force, raising the weight. 



An engine the converse of the ordinary steam or calorific 

 engines might thus be constructed, in which cold would be 

 apparently the motive power, and instead of a furnace supply- 

 ing the power, and the steam or air being abstracted by cold 

 water, ice might act as the initiating power, and warm 

 water be used as the abstractor. Even here the effects may 

 be viewed as caused by an abstraction of heat ; but they are 

 not the less the converse of the ordinary thermal effects. 

 Cold may be produced by mechanical force, e.g. the compres- 



