EEAT. 



493 



existence, they could not bo transformed into 

 each other This hypothesis being the very re- 

 verse of the fact, its dogmas have long offered a 

 barrier to the true course of physical investi- 

 gation. 



It is now established that the forces possess 

 none of the attributes of matter they are not 

 entities substantive things, endowed with pe- 

 culiar, persistent individual properties, but they 

 are modes of motion, or forms of movement in 

 common matter, and are convertible one into 

 another. It has long been known, for example, 

 that heat, as in the case of the steam engine, 

 produces mechanical force, while mechanical 

 force, as in the case of friction, produces heat. 

 But in what way is the effect related to the 

 cause ? The old hypothesis assumes the inter- 

 vention of a fluid, which, so long as its agency 

 is entertained, blinds us to the simplicity of the 

 facts. The new explanation says that the con- 

 ception of the fluid is superfluous that heat 

 actually passes into mechanical motion, and 

 mechanical motion actually passes into heat, or 

 that there is a conversion of one into another. 

 So with all the other forces known as " impon- 

 derables ; " they are mutually convertible into 

 one another a fact which has been described 

 by Mr. Grove, under the phrase " correlation 

 of forces." In his able treatise upon this sub- 

 ject, which, we are glad to learn, is to be repub- 

 lished in this country, he gives a lucid account 

 of the principle from which the following para- 

 graphs are abridged. 



The various affections of matter, which con- 

 stitute the main objects of experimental physics 

 namely, heat, light, electricity, magnetism, 

 chemical affinity, and motion, are all correla- 

 tive, or have a reciprocal dependence. Neither, 

 taken abstractly, can be said to be the essential 

 cause of the others, but either may produce or 

 be convertible into any of the others. Thus 

 heat may mediately or immediately produce 

 electricity, electricity may produce heat, and so 

 of the rest, each merging itself as the force it 

 produces becomes developed. The same must 

 hold good of other forces, it being an irresist- 

 ible inference from observed phenomena, that a 

 force cannot originate otherwise than by the 

 dissolution of some preexisting force or forces. 



The term correlation, strictly interpreted, 

 means a necessary mental or reciprocal depend- 

 ence of two ideas inseparable even in mental 

 conception ; thus the idea of height cannot ex- 

 ist without involving the idea of depth ; the 

 idea of pareat cannot exist without involving 

 the idea of offspring. The probability is that, 

 if not all, the greater number of physical phe- 

 nomena are correlative, and that without a du- 

 ality of conception the mind cannot form an 

 idea of them. Thus matter and force are cor- 

 relates in the strictest sense of the word ; the 

 conception of the existence of the one, involves 

 the conception of the existence of the other. 

 The correlation of forces implies their recipro- 

 cal production ; that any force capable of pro- 

 ducing another may, in its turn, be produced by 



it nay more, can be itself resisted by the force 

 it produces in proportion to the energy of such 

 production, as action is ever accompanied and 

 resisted by reaction. Thus the action of an 

 electro-magnetic machine is reacted upon by 

 the magneto-electricity developed by the action. 

 With regard to the forces of electricity and 

 magnetism in a dynamic state, we cannot elec- 

 trize a substance without magnetizing it we 

 cannot magnetize without electrizing it. Each 

 molecule, the instant it is affected by one of 

 these forces, is affected by the other, but in 

 transverse directions ; the forces are insepara- 

 ble and mutually dependent; correlative, but 

 not identical. 



In many cases where one physical force is 

 excited, all the others are also set in action. 

 Thus, when a substance, such as sulphuret of 

 antimony, is electrified, at the instant of elec- 

 trization, it becomes magnetic in directions at 

 right angles to the lines of electrical force ; at 

 the same time it becomes heated to a.n extent 

 greater or less according to the intensity of the 

 electric force. If this intensity is exalted to a 

 certain point, the sulphuret becomes luminous, 

 or light is produced ; it expands, consequently 

 motion is produced ; and it is decomposed, 

 therefore chemical action is produced. 



Motion, the most obvious, the most distinctly 

 conceived of all the affections of matter will 

 directly produce heat and electrieity, and 

 electricity, being produced by it, will produce 

 magnetism. Light also is readily produced by 

 motion ; either directly, as when accompanying 

 the heat of friction, or immediately by elec- 

 tricity resulting from motion. In the decom- 

 positions and compositions which the terminal 

 wires proceeding from the conductors of an 

 electrical machine develop when immersed in 

 different chemical media, we get the production 

 of chemical affinity by electricity, of which mo- 

 tion is the initial source. 



If heat be now taken as the starting point, 

 we shall find that the other modes of force may 

 be readily produced by it. Motion is so generally, 

 if it be not invariably, the immediate effect of 

 heat, that we may almost, if not entirely, resolve 

 heat into motion, and view it as a mechanical- 

 ly repulsive force tending to mane the particles 

 of all bodies, or to separate them from each 

 other. This molecular motion we may readily 

 change into the motion of masses, or motion in 

 its most ordinary and palpable form. Heat, 

 then, being a force capable of producing motion, 

 and motion, as we have also seen, being capa- 

 ble of producing the other modes of force, it 

 necessarily follows that heat is capable immedi- 

 ately of producing them. It will immediately 

 produce electricity, as shown in the beautiful 

 experiment of Seabeck. With regard to chemi- 

 cal affinity and magnetism, perhaps the only 

 method by which, in strictness, the force of 

 heat may be said to produce them, is through 

 the medium of electricity ; the thermo-electric 

 current being capable of deflecting the magnet, 

 of magnetizing iron, and exhibiting the other 



