Mr. J. Gill on the Dynamical Theory of Heat. Ill 



its orbit ; and also in applying the well-known principle of cen- 

 trifugal force of matter in masses to the phenomena of molecular 

 repulsion or elasticity in vapours and gases, by hypotheses of 

 molecular vortices which may hold good partially, but are not 

 sufficient to explain all the phenomena of this class which come 

 under our observation. The two great principles of energy or 

 force which seem to include all others, are attraction and repul- 

 sion'; and it might be more reasonably argued that, in the 

 majority of phenomena, motion is the result of these forces. 

 Perhaps the most probable supposition is that they were coex- 

 istent with motion from the creation of things. It must, how- 

 ever, be allowed that, as far as our observation extends, there is 

 a universal and immutable connexion between the motion of 

 matter or vis viva, and the more occult principles or properties 

 of attraction and repulsion in bodies at rest; so that whenever 

 motion or vis viva disappears, it is replaced by equivalent energy 

 under the form of attraction or repulsion in a statical condition 

 of disturbed equilibrium. And, conversely, every case of resto- 

 ration of disturbed statical equilibrium must give rise to equiva- 

 lent motion or vis viva. In other words, dynamical energy is the 

 force of motion, or vis viva ; and statical (or potential) energy is 

 a state of disturbed equilibrium of attraction or repulsion ; and, 

 as first clearly expounded by Mayer, these different forms of 

 energy are convertible. Heat, as we observe it in common 

 matter, is allowed to be molecular motion, and the calorific 

 energy of the hottest flame to be only the vis viva of the excited 

 material particles. As heat is the proximate cause of work in 

 thermic prime movers, it is reasonable to suppose that all the 

 energy corresponding to the work done by a steam-engine must 

 pass from the fire into the steam, and from the steam to the 

 work done. The heat required to form the steam is allowed to 

 be constant, or nearly so, under different pressures ; and one cubic 

 foot of steam under a pressure of ten atmospheres, quietly con- 

 densed in the vessel which contains it, will communicate to the 

 condensed water only the same quantity of heat as ten cubic feet 

 of steam of atmospheric pressure equally condensed. If the 

 cubic foot of high-pressure steam be allowed to expand gradu- 

 ally, by enlarging the space it occupies, against a moderated 

 resistance until its bulk becomes ten cubic feet, supposing no 

 heat to be lost or applied in the process, it is perceived that it 

 will be identical with the ten cubic feet of steam formed at atmo- 

 spheric pressure; but expansion under moderated resistance 

 necessarily produces work, and, in fact, the work produced by 

 the high-pressure steam would be three times more than the low- 

 pressure steam can produce. The energy equivalent to this 

 remarkable difference of amount of work must exist in the high- 



