276 LATENT HEAT AN EXPANSIVE FORCE. SECT. XXVII. 



mere transfer of force, in which the force of compression is 

 exactly proportional to the force of expansion. Ice remains at 

 the temperature of 32 Fahrenheit till it has absorbed 140 of 

 heat, and then it melts, but without raising the temperature of 

 the water above 32. On the contrary, when a liquid is con- 

 verted into a solid, a quantity of heat leaves it without any 

 diminution of temperature. Thus water at 32 must part with, 

 140 of heat before it freezes. The slowness with which water 

 freezes or ice thaws, is a consequence of the time required for 

 the ethereal atmospheres round the particles of the water to 

 contract or expand with a force equivalent to 140 of heat. A 

 considerable degree of cold is felt during a thaw, because the 

 ice in its transition from a solid to a liquid state absorbs 

 sensible heat from the atmosphere and surrounding objects. The 

 heat absorbed and evolved by the rarefaction and condensation 

 of air is exactly proportional to the force evolved and absorbed 

 in these operations. In fact, the changes of temperature pro- 

 duced by these rarefactions and condensations of air show that 

 the heat of elastic fluids is the mechanical force possessed by 

 them ; and since the temperature of a gas determines its elastic 

 force, it follows that the elastic force or pressure must be the 

 effect of the motion of the constituent particles in any gas. 

 Sir Humphry Davy, who first demonstrated the immateriality of 

 heat, assumed the hypothesis that the motion we call heat is a 

 rotation or vibration among the particles of the fluid, which, ac- 

 cording to Mr. Joule, agrees perfectly with the observed pheno- 

 mena, but he prefers the more simple view pf Mr. Herapath, that 

 the elastic force orpressure is due to the impact of the particles 

 against any surface presented to them. Absorbed or latent heat 

 may be regarded as a quiescent energy ready to be restored to the 

 form of sensible heat when called forth : its vibrations as heat 

 are extinguished for the time by being transferred to the internal 

 expansive force, and are restored by compression. The absorbed 

 heat of air and all elastic fluids may be forced out by sudden 

 compression like squeezing water out of a sponge. The quantity 

 of heat brought into action in this way is well illustrated by the 

 experiment of igniting tinder by the sudden compression of air 

 by a piston thrust into a cylinder closed at one end. The 

 development of heat on a stupendous scale is exhibited in 

 lightning: it is proportional to the square of the quantity 



