THE STORY OF NINETEENTH-CENTURY SCIENCE 



between the molecules themselves. This potential en- 

 ergy (which will be recovered when the gas contracts) is 

 the " latent heat " of Black, which so long puzzled the 

 philosophers. It is latent in the same sense that the en- 

 ergy of a ball thrown into the air is latent at the mo- 

 ment when the ball poises at its greatest height before 

 beginning to fall. 



It thus appears that a variety of motions, real and po- 

 tential, enter into the production of the condition we 

 term heat. It is, however, chiefly the translational mo- 

 tion which is measurable as temperature ; and this, too, 

 which most obviously determines the physical state of 

 the substance that the molecules collectively compose 

 whether, that is to say, it shall appear to our blunt per- 

 ceptions as a gas, a liquid, or a solid. In the gaseous 

 state, as we have seen, the translational motion of the 

 molecules is relatively enormous, the molecules being 

 widely separated. It does not follow, as we formerly 

 supposed, that this is evidence of a repulsive power act- 

 ing between the molecules. The physicists of to-day, 

 headed by Lord Kelvin, decline to recognize any such 

 power. They hold that the molecules of a gas fly in 

 straight lines in virtue of their inertia, quite indepen- 

 dently of one another, except at times of collision, from 

 which they rebound in virtue of their elasticity ; or an 

 approach to collision, in which latter case, coming with- 

 in the range of mutual attraction, two molecules may 

 circle about one another, as a comet circles about the 

 sun, theii rush apart again, as the comet rushes from 

 the sun. 



It is obvious that the length of the mean free path of 

 the molecules of a gas may be increased indefinitely by 

 decreasing the number of the molecules thernselves in a 



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