FOR THE ADVANCEMENT OP SCIENCE, 1857. 503 



tion to subsist between the interacting forces, so that if one were 

 doubled or trebled in amount, the other should undergo a propor- 

 tionate change ? This anticipation, it has been already stated, has 

 been realized by Mayer and Joule. The discovery of the mechanical 

 equivalent of heat has been rapidly followed by that of other 

 forces ; and we now know not only that electricity, magnetism, and 

 chemical action, in given quantities, will produce each a definite 

 amount of mechanical work, but we know further chiefly through 

 the labours of Mr. Joule what that relation is, or, in other words, 

 the mechanical equivalent of each force. 



The first step in this important career of discovery though 

 long unperceived in its relation to the rest was, undoubtedly ? 

 Faraday's proof of the definite chemical effect of the voltaic cur- 

 rent. The last will probably be to reduce all these phenomena to 

 /nodes of motion, and to apply to them the known principles of 

 dynamics, in such a way as not only to express the laws of each 

 kind of movement, as it is in itself, but also the connexion and 

 dependence of the different classes of the phenomena. 



A bold attempt at such a generalization has been made by 

 M. Helmholtz. The science of Thermo-dynamics starts from the 

 principle, that perpetual motion is impossible, or, in other words, that 

 we cannot, by any combination of natural bodies, produce force out 

 of nothing. In mechanical force, this principle is reducible to the 

 known law of the conservation of living force ; and M. Helmholtz 

 has accordingly endeavoured to show that this law is maintained in 

 the interaction of all the natural forces ; while, at the same time, 

 the assumption of its truth leads to some new consequences in 

 physics, not yet experimentally confirmed. Expressed in its most . 

 general form, this principle asserts that the gain of vis viva during the 

 motion of a sj stern is equal to the force consumed in producing it 

 from which it follows, that the sum of the vires vivce, and of the 

 existing forces, is constant. This principle M. Helmholtz deno- 

 minates the conservation of force. A very important consequence of 

 its establishment must be, that all the actions of nature are due to 

 attractive and repulsive forces, whose intensity is a function of 

 the distance, the conservation of vis viva holding only for such 

 forces. 



It is usually stated, in mechanical works, that there is a loss of 

 vis rit'a in the collision of inelastic bodies, and in friction. This 

 is true with respect to the motion of masses, which forms the sub- 



