14 Mr. W. J. M, Rankine on the Mechanical Action of Heat. 



The first factor, „ ^j. , may be considered in general as a known 



quantity ; for C represents, as already stated, 274*6 Centigrade 

 degrees, the absolute temperature of melting ice, and nM the theo» 

 retical weight, in the perfectly gaseous state, of unity of volume 

 of the substance, under unity of pressure, at that temperature ; 



or what is the same thing, -^ is the height of an imaginary 



column of the substance, of uniform density, and at the tempe- 

 rature of melting ice, whose pressure by weight upon a given 

 area of base is equal to its pressure by elasticity, supposing it 



3kM. 

 to be perfectly gaseous. The determination of the ratio -^ — 



is necessary, to complete the solution of the problem. 



5. The relation now to be investigated between heat and me- 

 chanical power, is that which exists between the power expended 

 in compressing a body into a smaller volume, and the increase 

 of heat in consequence of such a compression, and conversely, 

 between the heat which disappears, or, as it is said, becomes 

 latent, during the expansion of a body to a greater volume, and 

 the mechanical power gained or developed by that expansion. 

 Those phsenomena, according to the hypothesis now under con- 

 sideration, as well as every hypothesis which ascribes heat to 

 motion, are simply the transformation of mechanical power from 

 one shape into another. 



It is obvious, in the first place, without the aid of algebraical 

 symbols, that the general effect of the compression of an oscilla- 

 ting atomic atmosphere, or molecular vortex, must be to accele- 

 rate its motion, and of its dilatation, to retard its motion ; for 

 every portion of such an atmosphere is urged towards the nucleus 

 or atomic centre by a centripetal force equal to the centrifugal 

 force arising from the oscillation ; so that when, by compression, 

 each portion of the atmosphere is made to approach the centre 

 by a given distance, the vis viva of its motion will be increased 

 by the amount corresponding to the centripetal force acting 

 through that distance ; and conversely, when by expansion each 

 portion of the atmosphere is made to retreat from the centre, the 

 vis viva of its motion will be diminished by a similar amount. 



It is not, however, to be taken for granted, that all the power 

 expended in compressing a body appears in the form of heat. 

 More or less power may be consumed or developed by changes 

 of molecular arrangement, or of the internal disti-ibution of the 

 density of the atomic atmospheres ; and changes of molecular 

 arrangement or distribution may develope or consume heat, in- 

 dependently of changes of volume. 



6. We shall now investigate, according to the hypothesis of 



