THE VITAL AND PHYSICAL FORCES. 
731 
without any hypothetical assumption, he would state his idea of the “correlation” of 
two forces, A and B, to be this ; — that A, operating upon a certain form of matter, 
ceases to manifest itself, but that B is developed in its stead ; and that, vice versa, B, 
operating upon some other form of matter, ceases to manifest itself, but that A is re- 
produced in its stead. The idea of correlation also involves that of a certain definite 
ratio or equivalent between the two forces thus mutually interchangeable ; so that 
the measure of force B, which is excited by a certain exertion of force A, shall, in its 
turn, give rise to the same measure of force A as that originally in operation. Thus, 
when an electric current is set in motion (to use the common phraseology) by gal- 
vanic action, the amount of chemical decomposition which it will effect bears a pre- 
cise correspondence {cceteris paribus) with the amount of zinc which has undergone 
oxidation ; chemical action thus exciting electricity, which in its turn reproduces the 
original equivalent of chemical action. In like manner, when water at 212° is con- 
verted into steam, the heat which it receives is no longer manifested as heat, but 
mechanical force is developed in its stead, and this in a certain definite ratio ; as 
soon, however, as the steam, losing its elasticity by condensation, returns to the 
condition of water, the original equivalent of heat is again developed, its mechanical 
force being no longer manifested. 
Whether we regard it as most consonant to our ideas of the nature of force, to con- 
sider the one force, in any such case, as itself becoming latent, whilst it excites an 
equivalent measure of a force of another kind which was previously dormant , — or 
whether we consider that the one force is actually converted into the other, and that 
there is really no such condition as dormant or latent force, — the fact of the mutual 
relationship, and the definite character of that relationship, remains the same ; and 
it is upon this, rather than upon any hypothetical representation of it, that the author 
wishes chiefly to insist. Although, therefore, the terms “ conversion ” and “ meta- 
morphosis” will be occasionally employed in the present paper, as the most convenient 
modes of expressing the author’s meaning, he is desirous that it should be understood 
that he does not desire to imply anything else than the existence of the relationship 
just defined. 
One more preliminary remark is necessary, upon a point on which Prof. Grove has 
not thought it requisite strongly to dwell ; namely, the necessity for a certain material 
substratum as the medium of the change in question. Thus, to take a familiar case, 
the correlation of Electricity and Magnetism is indicated by the development of 
magnetic attractions and repulsions in iron, when a current of electricity is made to 
circulate around it. In like manner, the correlation between Heat and Electricity is 
shown in the disturbance of electric equilibrium, which ensues on the application of 
heat to bars of certain dissimilar metals (especially bismuth and antimony) in con- 
tact with each other. The iron, in the first case, is the necessary medium for the 
development of the magnetic force by electricity ; as the bars of dissimilar metals are 
in the second, for the development of the electric force by heat. So, again, in the 
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