196 ANNUAL OF SCIENTIFIC DISCOVERY. 



was mutual ; and that of any two, compared together, either may stand to 

 the other in the relation of cause. With respect to heat and mechanical 

 force, this has been long known. When a body is compressed by mechani- 

 cal force, it gives out heat ; and, on the other hand, when it is healed, it di- 

 lates, and evolves power. The knowledge of the action of electricity in dis- 

 solving the bonds of chemical union followed closely upon that of the in- 

 verse phenomenon ; and the discovery of electro-magnetism by Oersted was 

 soon followed by that of mayneto-electridty by Faraday. With reason, there- 

 fore, it occurred to many minds that the relations of any two of the forces 

 of nature were mutual that that which is the cause, in one mode of inter- 

 action, may become the effect, when the order of the phenomena is changed; 

 and that, therefore, in the words of Mr. Grove, one of the able expound- 

 ers of these views, while they are " correlative," or reciprocally dependent, 

 " neither, taken abstractly, can be said to be the essential cause of the oth- 

 ers." But a further step remained to be taken. If these forces were not 

 only related, but mutually related, was it not probable that the relation was 

 also a definite one ? Thus, when heat is developed by mechanical action, 

 ought we not to expect a certain definite proportion to subsist between the 

 interacting forces, so that if one were doubled or trebled in amount, the 

 other should undergo a proportionate 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 labors 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 

 great discovery of the definite chemical effect of the voltaic current. The 

 last will probably be to reduce all these phenomena to modes 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, and also 

 the connection 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 Thermodynamics 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 vis viva ; and 

 M. Helmholtz has accordingly endeavored to show that this law is main- 

 tained 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 system, 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 princi- 

 ple M. Helmholtz denominates the conservation of force. A very important 

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



