certain restrictions; the ground of the remarkable persistency and permanence of well-constituted species; a general harmony and homology throughout all creation: proceeding to illustrate its action on the mental or spiritual world; accounting for perception, remembrance, reasoning, imagining, judging, and upon all our other modes of mental activity, as operations of the cosmical law of assimilation. In the world of physics he gives, as illustrations of this same law of assimilation, attraction, inertia, elasticity, heredity, reversion, symmetry culminating in sphericity or symmetrical cellularity, chemical and electrical action; especially in voltaic action the influence and the persistence of this law is most remarkably displayed. By the way of familiar illustration, he takes the original voltaic cell, and without attempting to explain how one solid, copper or platinum, comes to be less assimilable to a liquid than another, such as zinc, he shows that just what we are to expect, from this law of assimilation, takes place--viz., that at the zinc there tends to form a stratum of oxygen, and that, at the platinum, there tends to form a stratum of hydrogen. Pursuing the old view as to the cause of the state of tension induced in the dilute sulphuric acid, the continuous decomposition of the water, the solidifying action of the zinc surface, he confines his attention to the current of force instituted by the oxygen; advancing the idea that this is not merely force in general, of which all that is to be considered is its quantity and direction; but force, of which the form of its elements or their formative power is also to be considered;--that formative power being representative or productive of oxygen. To the objection that such a conception is occult and mysterious, he asks if it is more occult and mysterious than what is implied and confessed to be hid under the term electricity, or in the phenomena of heredity, or than anything else which is adduced as a cause of a particular phenomenon. The cosmical law of assimilation explains all these phenomena, and, without any special hypothesis, is precisely what is wanted, in order to render natural knowledge as a whole accessible to the student: something which puts him in possession, from the first, of a master-thought, which, if he carry it along with him, will present all nature as a harmony; explaining all that stands in need of explanation. Macvicar continues:--