HEAT. 63 



sion of water between 32 and 39, or by the expansion of 

 gases. It seems difficult to ascertain that in any experiments 

 some cold is not produced by elongation, expansion, or other 

 molecular change, which may neutralise some of the heat 

 produced. To call the work done by ice in cooling water 

 a mechanical effect of heat, the calorific sign must be 

 changed ; and though the phenomena may be entirely con- 

 sistent with the conservation of force, and no force may be 

 created or destroyed, the work done by the water, appa- 

 rently expanded by cold, is in the same direction to that 

 which is effected by heat, when the substance is above or 

 below the given temperature. I have stated in previous 

 editions of this work that ' Although, taking the phenomena 

 as they are known to exist, the mechanical laws may be 

 deduced, yet in any physical conception of the nature of heat 

 expansion by cold has always been a great stumbling-block 

 to me, and, I believe, to many others ;' and, although it may 

 be true, that, assuming a given amount of mechanical force to 

 be all converted into heat, or a given amount of heat into 

 mechanical force, there must be a relation of equivalence, I 

 feel by no means certain that the true equivalent of heat for 

 arrested motion has yet been attained. Mechanical force is 

 now used in the arts for producing cold, and although it 

 doubtless at the same time produces heat, the cold, if it be 

 not equivalent to the heat, has to be deducted or estimated in 

 calculating the result It may turn out that in some cases it 

 is a more correct expression to say that mechanical force pro- 

 duces a disturbance of the equilibrium of temperature than 

 that it produces heat. 



Molecular vibrations also, which cannot be called heat (just 

 as undulations of a certain slowness or rapidity cannot be 

 called sound or light), carry off some of the force employed, 

 and electrical currents or inductive effects may convey away 

 some, so that to say that a given dynamic force has, in any 

 experiments hitherto made, been entirely converted into heat, 

 and heat only, is a proposition of which, to my mind at least, 

 there is not conclusive evidence. 



To make it sure that a given motion of a given body has 

 all been converted into heat, the motion itself should be 



