NATURAL PHILOSOPHY. 197 



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 viva in the 

 collision of inelastic bodies, and in friction. This is true with respect to the 

 motion of masses, which forms the subject of mechanical science as at pres- 

 ent limited ; but it is not true in a larger sense. In these, and such-like 

 cases, the movement of masses is transformed into molecular motion, and 

 thus re-appears as heat, electricity, and chemical action ; and the amount 

 of the transformed action definitely corresponds to the mechanical force 

 which was apparently lost. In the cases just considered, mechanical action 

 is converted into molecular. But molecular actions of different kinds are 

 themselves, in like manner, interchangeable. Thus, when light is absorbed, 

 vis viva is apparently lost ; but not to speak of phosphorescence, in which 

 the light absorbed, or a portion of it, is again given out in all such cases 

 heat and chemical action are developed, and in amount corresponding to the 

 loss. Hence the apparent exceptions to the principle are in reality confir- 

 mations of it ; and we learn that the quantity of force in nature is as un- 

 changeable as the quantity of matter. This, however, is not true of the 

 quantity of available force. It follows from Carnot's law, that heat can be 

 converted in mechanical work only when it passes from a warmer to a colder 

 body. But the radiation and conduction by which this is effected tend to bring 

 about an equilibrium of temperature, and therefore to annihilate mechanical 

 force : and the same destruction of energy is going forward in the other 

 processes of nature. Thus, it follows from the law of Carnot, as Prof. 

 Thomson has shown, that the universe tends to a state of eternal rest ; and 

 that its store of available force must be at length exhausted. Mr. Rankine 

 has attempted, in another method, to combine the physical sciences into one 

 system, by distinguishing the properties which the various classes of physi- 

 cal phenomena possess in common, and by taking for axioms propositions 

 which comprehend their laws. The principles thus obtained are applicable 

 to all physical change ; and they possess all the certainty of the facts from 

 which they are derived by induction. The subject matter of the science so 

 constituted is energy, or the capacity to effect changes ; and its fundamental 

 principles are, first, that all kinds of energy and work are homogeneous or 

 in other words, that any kind of energy may be made the means of performing 

 any kind of work ; and, secondly, that the total energy of a substance can- 

 not be altered by the mutual action of its parts. From these principles the 

 author has deduced some very general laws of the transformation of energy, 

 which include the known relations of physical forces, Dr. Loyd's Address 

 before the British Association, 1857. 



ON THE DEVELOPMENT OF HEAT IN AGITATED WATER. 



Mr. G. Rennie, in a communication to the British Association on the 

 above subject, stated that the subject of the mechanical or dynamic force re- 

 quired to raise a given quantitity of water one degree of Fahrenheit had 

 long been the object of the research of philosophers, ever since Count Rum- 

 ford, in his celebrated experiments on the evolution of heat in boring guns 



17* 



