HEAT. 41 



moves this to a point at which the tension or elastic force of 

 the confined air equals that of the surrounding air. If the 

 confined air be kept at this point, the piston would remain 

 stationary ; but if it be cooled, the external air exercising 

 then a greater relative degree of pressure, the piston returns 

 towards its original position ; just as it will be seen, when we 

 come to the magnetic force, that a magnet placed in a partic- 

 ular position produces motion in iron near it, but to make 

 this motion continuous, or to obtain an available mechanical 

 power, the magnet must be demagnetised, or a stable equili- 

 brium 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 ; nor do 

 we, 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 measur- 

 ers 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 moved. There are, in- 

 deed, some delicate experiments which tend to prove that a 

 repulsive action between separate masses is produced by heat. 

 Fresnel found that mobile bodies heated in an exhausted re- 

 ceiver repelled each other to sensible distances ; and Baden 

 Powell found that the coloured rings usually called Newton's 

 rings change their breadth and position, when the glasses be- 

 tween which they appear are heated, in a manner which 

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

 ory 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 



