THE CONSTANCY OF MOTION. 229 



Yet another example. Let us take the case of a cylinder of com- 

 pressed air which for years, tightly sealed, may serve as a store of 

 force. 



Prof. Clerk Maxwell explains how an ounce of air, in a closed and 

 fragile jar, sustains the outside pressure of the atmosphere amounting 

 to several tons ; this he does by the theory that the ounce of air is 

 made up of molecules which have so rapid a motion among themselves 

 that they collide on the inside of the jar with as great a force as that 

 of the atmospheric pressure externally. 



This theory, now widely accepted, rests on the solid grounds of 

 the measured velocity of air rushing into a vacuum, which is the 

 same as that assumed for its internal motion ; and further, on some 

 observations of the diffusion of various gases into each other, which it 

 would be out of place to detail here. 



On the basis, then, of Prof. Maxwell's theory, we can believe that, 

 in a cylinder of compressed air, the energy stored up exists in no 

 merely " potential " form, but in the full actuality of the rapid motion 

 of gaseous particles, which may take the shape of mass-motion when 

 the piston is allowed to move outward. 



An extension of the hypothesis that motion is continuous would 

 lead to the inference that the so-called latent heat of water at 32 

 Fahr., as compared with ice at the same temperature, is due to the 

 swifter movement of the molecules in the former case ; and no facts 

 are better known than that mechanical motion can become heat, and 

 that heat turns ice into water. 



Another implication of this theory is that atoms, as free hydrogen, 

 oxygen, or carbon, are in exceedingly greater commotion than mole- 

 cules, such as those of water or carbonic-acid gas. For the decompo- 

 sition of such molecules may be effected by the exhaustion of mechani- 

 cal motion, and I take it that the dynamic state of the products must 

 balance this expenditure. What else can become of it ? 



The measure of the contained motion in two different atoms can 

 be noted on their combining chemically, by the increase of tempera- 

 ture, which has its well-known mechanical equivalent. 



The analogy between chemical units and small magnets is very 

 close: as a magnet decreases in size, so, relatively thereto, does its 

 attractive power increase ; and, were we able to go on dividing 

 until we came to a single atom of iron, we should doubtless have 

 the magnetism merge into the equivalent phase of intense chemical 

 affinity. 



Without further illustration, then, of the principle set forth, I will 

 say that, as in recent years we have had to familiarize ourselves with 

 the idea that many forms of actual energy are impalpable, as the rays 

 of light and the waves of sound, so now I think there are good 

 grounds for extending our ideas so as to believe that all phases of 

 "potential" energy are really actual ; that, as -we cannot but think 



