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



available from the direct pressure arising from the tempo- 

 rary heat of the explosive action, even when the bulk of the 

 resulting gases is not, after cooling, greater than that of their 

 constituents, — as experiment shows that in some such cases a 

 momentary increase of bulk of 14 or 15 times the volume when 

 cold may result, indicating a temperature of at least 7000° F. 

 But in obtaining motive power from fire, the molecular motion 

 of the flame is generally transmitted to water in a closed boiler, 

 where, by increasing the intestine movements of the liquid par- 

 ticles, it raises the temperature until, at a point depending on 

 the superincumbent pressure, the particles in more immediate 

 contact with the source of heat undergo a sudden change of condi- 

 tion, and, absorbing instantaneously a large additional amount of 

 heat (at the ordinary temperature of ebullition), assume the state 

 of vapour and rise to the surface. Under a pressure of 1700 

 atmospheres water might be heated to about 1200° F. (or 1000° 

 above the common boiling-point), in which condition it would 

 contain heat enough to convert the whole mass into steam, the 

 available amount of repulsion being in proportion ; so that by 

 expanding down to atmospheric pressure against a resistance 

 gradually decreasing from 1700 atmospheres to 1, it might per- 

 form a quantity of work eight or ten times as great as is gene- 

 rally obtained from the best steam-engines. This enormous 

 amount of potential energy stored up in the superheated water 

 in the shape of highly excited repulsion, would seem to be due 

 more to the concentration of the heat in possession than to any 

 excess of quantitative heat which the hot fluid may be supposed 

 to contain above the heat of conversion of the same weight of 

 steam at atmospheric pressure. 



Energy may be put into an elastic fluid, or rather the fluid 

 can be put into a condition of increased energy, either by com- 

 pression or by the actual addition of heat ; and in both cases the 

 quantity of sensible heat, or the temperature, is increased. I 

 have before expressed my opinion that compression, apart from 

 the idea of molecular friction, is not a source of quantitative 

 heat, but only of concentration of heat or temperature. In the 

 act of compression the particles are forced closer together ; and 

 supposing the total quantity of heat to remain unchanged, the 

 increase of temperature may result from concentration of mo- 

 lecular action as to the number of vibrations made by each 

 particle in a given time. By the actual addition of heat to 

 an elastic fluid under constant volume, the same effect may be 

 produced of increasing the number of molecular vibrations in a 

 given time. In both cases the heat-tone may be supposed to 

 become more acute from a corresponding change of the thermic 

 rhythm of the molecular vibrations. In compression the increased 



