AND THE NATURE OF FORCE. 



493 



In answer to the question, What is understood by the doctrine of 

 conservation ? a better answer cannot be given than the following, 

 extracted from a book called "Faraday as a Discoverer," and appended 

 by Prof. Tyndall to his treatise, " Heat a Mode of Motion." It is as 

 follows : " Of the inner quality that enables matter to attract matter, 

 we know nothing ; and the law of conservation makes no statement 

 regarding that quality. It takes the facts as they stand, and affirms 

 only the constancy of working power. That power may exist in the 

 form of MOTION ; or it may exist in the form of force, with distance 

 to act through. The former is dynamic energy, the latter is potential 

 energy ; the constancy of the sum of both being affirmed by the law 

 of conservation. The convertibility of natural forces consists solely 

 in transformations of dynamic into potential, and of potential into 

 dynamic energy, which are incessantly going on. In no other sense 

 has the convertibility of force at present any scientific meaning." 



The following quotation from Tyndall's "Heat a Mode of Motion," 



will illustrate the application of the principle to a special case : — 



" Suppose a certain amount of heat to be imparted to this lump of 



lead, how is that heat disposed of within the substance? It is 



applied to two distinct purposes : it performs two difierent kinds of 



work. One portion of it excites that species of motion which 



augments the temperature of the lead, and which is sensible to the 



thermometer ; but another portion of it goes to force the atoms of 



lead into new positions, and this portion is lost as heat. The pushing 



asunder of the atoms of the lead in this case, in opposition to their 



mutual attractions, is exactly analogous to the raising of our weight 



in opposition to the force of gravity, a loss of heat in both cases 



being the result. Let me try to make the comparison between the 



two actions still more strict. Suppose that a definite amount offeree 



is to be expended upon our weight, and that this force is divided into 



two portions, one of which is devoted to the actual raising of the 



weight, while the other is employed to cause the weight as it ascends 



to oscillate like a pendulum, and to oscillate moreover with gradually 



augmented width and rapidity ; we have then the analogue of that 



which occurs when heat is imparted to the lead. The atoms are 



pushed apart, but during their recession they vibrate with gradually 



augmented intensity. Thus, the heat communicated to the lead 



resolves itself in part into atomic potential energy, and in part into 



actual energy, which may be regarded as a kind of atomic music. 



