HEAT. 29 



this motion continuous, or to obtain an available mechanical 

 power, the magnet must be demagnetised, or else a stable 

 equilibrium is obtained. 



In the case of the piston moved by heated air, the motion 

 of the mass becomes the exponent of the' amount of heat 

 i.e. of the expansion or separation of the molecules ; and we 

 do not, by any of our ordinary methods, test heat in any other 

 way than by its purely dynamical action. The various modi- 

 fications of the thermometer and pyrometer are all measurers 

 of heat by motion ; in these instruments liquid or solid bodies 

 are expanded and elongated, i.e. moved in a definite direction, 

 and, either by their own visible motion, or by the motion of 

 an attached index, communicate to our senses the amount of 

 the force by which they are moved. There are, indeed, some 

 delicate experiments which tend to prove that a repulsive 

 action between separate masses is produced by heat. Fresnel 

 thought he had proved that mobile bodies heated in an 

 exhausted receiver repelled each other to sensible distances ; 

 but his experiments have not been confirmed ; and Baden 

 Powell found that the coloured rings usually called Newton's 

 rings change their breadth and position, when the glasses 

 between which they appear are heated, in a manner which 

 showed that the glasses repelled each other. M. Faye's theory 

 of comets is based on "some such repellent force. There is, 

 however, some difficulty in presenting these phenomena to 

 the mind in the same aspect as the molecular repulsive action 

 of heat. 



The phenomena of what is termed latent heat have been 

 generally considered as strongly in favour of that view which 

 regards heat either as actual matter, or, at all events, as a 

 substantive entity, and not a motion or affection of ordinary 

 matter. 



The hypothesis of latent matter is, I venture with diffidence 

 to think, a dangerous one it is something like the old prin- 

 ciple of Phlogiston : it is not tangible, visible, audible ; it is, in 

 fact, a mere subtle mental conception, and ought, I submit, 

 only to be received on the ground of absolute necessity, the 

 more so as these subtleties are apt to be carried on to other 

 natural phenomena, and so they add to the hypothetical 



