Chemical affinity, or the force by which dissimilar bodies tend to unite and form compounds differing generally in character from their constituents, will directly produce motion of definite !names, by the resultant of the molecular changes it induces, of which the projectile effects of gunpowder are familiar instances. By chemical affinity, again, we can directly produce electricity, and through its medium it may be quantitatively converted into the other modes of force : but heat and light are immediate products of chemical affinity, and che mical action produces mognetism whenever it is thrown into a definite direction, as in the phenomenon of electrolysis, a simple instance of which Mr. Grove adduces as presented by his gas voltaic battery. This completes the review of the phenomena arising from the correlation of the physical forces, each taken in turn as the initiating one.
Mr. Grove states his belief that the same principles and mode of reasoning as have been adopted in his Essay, might be applied to the organic as well as the inorganic world ; and that muscular force, animal and vegetable hest, &c., might, and at some time will, be shown to have similar definite correlations ; but that he has purposely avoided this subject, as pertaining to a department of science to which lie has not devoted his attention. lie alludes, however, to the experiments of Professor Matteuci, by .which it appears that whatever mode of force it be which is propagated along the nervous filaments, that mode of force is definitely affected by currents of electricity ; and ho states, also, that by an application of the doctrine of the correlation of forces, Dr. Carpenter has shown bow a difficulty arising from the ordinary notions of the development of an organised being from its germ-cell may be lessened.
In the Philosophical Transactions' for 1850, pp. 727.757, Is a valuable paper by Dr. Carpenter ' On the Mutual Relations of the Vital and Physical Forces, in which the principle of the correlation of the latter is successfully extended in some detail to the former, the subject of the germ-cell considered, as just alluded to, and, the view of the correlation of the nervous and electrical forces, in particular, which had been proposed by Dr. Carpenter someyears before, and had been afterwards formally adopted by Professor Matteuci, fully established. In this paper, perhaps the most impor tant general enunciation on the subject of correlation which has followed Mr. Grove's' Essay,' and certainly the first, of an adequately definite and compthheusive nature, on the application of the doctrine to the phenomena of organisation, the author etates, that he "is not aware that any other attempt has been made to furmularise the entire series of these mutual relations, [the 'very intimate mutual relations 'of the physical forces) than that which has been put forth by Professor Grove in his short, treatise On the Correlation, &c." But Mr. Grove, wo must hero remark, did much more than merely forinularise these relations. He announced a new conception of their nature, which logically speakin', is a unircraal of physics, and this it is which gives his enunciation its true value, and elevates it almost to the rank of a discovery. Another of Dr. Carpenter's introductory remarks concerns "a point on which," he says, "Professor Grove has not thought it requisite strongly to dwell ; namely, the necessity for a certain material substratum as the medium" of the conversion of force. The existence, however, of that substratum is a necessary implication of Mr. Grove's
entire doctrine. Ae he considers all the physical forces to be intrin sically affections of matter,—underatanding that term (to use the language of philosophical criticism) in its vulgar eense,—the necessity in question is an inseparable clement of his theory, and is therefore virtually dwelt upon throughout his Essay.
Dr. Carpenter has lately (Feb. 24, 1860) returned to this subject, in I a discourse delivered at the Royal Institution, and printed in the at the meetings of ti e members, vol. iii., pp. 206-209, On the Relation of the Vital to the Physical Forces.' Heargues, that when we carefully look into the question, we find that what the germ really supplies is not the force but the directive agency. This agency may he regarded, he observes, like magnetism, as a static force ; and just as magnetism requires to be (as we have already seen from Mr. Grove) combined with motion to enable it to develops electricity, so does the directive agency of the germ need the co-operation of a dynamic force for the manifestation of its organising power. That dynamic force is heat, the influence of which upon the rate of growth and development, both animal and vegetable, is so marked as to have universally attracted the attention of physiologists, who, however, have only recognised in it a vital stimulus, calling forth the latent power of the germ, instead of looking upon it as itself furnishing the power that does the work. It is indeed only when the physiological survey is extended from the vital phenomena of warm-blooded animals to those of cold-blooded animals and of plants, that the immediate and direct relation between heat and vital energy, as manifested in the rate of growth and development, or of other changes peculiar to the living body, is unmistakeably manifested. " Hence," Dr. Carpenter con cludes, after summarily reviewing the phenomena of vegetable life tinder this point of view, " we seem justified in affirming that the correlation between heat and the vital force of plants is not less intimate than that which exists between heat and motion. The special attribute of the vegetable germ is its power of effecting the metamorphosis, and of utilising the organising force according to the plan of construction characteristic of each species." For the details of Mr. Grove 'a own illustrations and applications of his doctrine, we must refer to his work itself, in which, also, are given his views of the intrinsic nature of the physical forces ; views which are inseparable, in his opinion, from the dogma of their correlation. In this, however, we cannot agree with him. That dogma, we con ceive, will remain a concrete expression of the truth, as a first approx. inuaion of great generality, and in point of time the first approximation to a general theory, whatever notion may be finally established of the nature of the forces concerned, and whether heat, for instance, be regarded as a motion of the particles of ordinary matter—that is of the matter exhibiting its phenomena—or as a motion of a calorific ether pervading that matter. This discrimination we believe will be found important in the further application and investigation of the subject. In the article GROVa, WILLIAll ROBERT, in the BIOORAPIIICAL DIVISION of this work, some remarks relevant on this head, have been offered with respect to the history and interpretation of the principle of the Correlation of Physical Forces, to which we may now refer as com pleting, together with the present and following remarks, our view of the subject.