“In the Philosophical Magazine the reader will find a series of Essays by Mr. Allen; rich in ingenuity of argument, and abounding in masterly views on the great subject of chemical agency, as affecting changes in the modes of existing of physical matter. These essays all go upon the principle, that in every change of existence that matter is capable of undergoing, caloric is given out or absorbed in the form of either electricity, of galvanism, of caloric, or of light. Respecting the important question which has recently agitated the philosophical world, and which has been proposed as a prize in one of the Societies abroad; viz. In what does the difference consist between galvanism and electricity? Mr. Allen observes, ‘In electricity we contrive, by mechanical means, to collect the loose and uncombined quantity from the earth and surrounding medium; and this we do in circumstances in which it has nothing to act upon, as free from moisture of any kind as possible; in fact, from every thing readily soluble in heat or in this power. I would therefore, he says, define electricity to be the object of science which treats of the mechanical and natural means of separating this _grand agent_ from some of its combinations, and of ascertaining its actions in this state.’ ‘In galvanism, on the other hand, this solvent power, this electric fire, is produced in circumstances in which it has _substances_ to act upon; substances which are most readily dissolved in it; substances, in fact, which seem to form the grand medium between this _power and passive substances_, and which are partially dissolved in it. And hence I define galvanism as the electric fire, or _grand agent_, only _partially_ separated from its combinations; by which I refer principally to oxygen and hydrogen.’ After illustrating this principle, by referring to the circumstances in which the chemical agency of galvanism appears more conspicuous than that of electricity, he adds, ‘thus we perceive, that when _the grand agent of nature_ is _more perfectly_ separated from its combinations it is ELECTRICITY; when partially separated, GALVANISM.’ Of these views and principles we have a more ample illustration and defence as the author proceeds in his investigation; and the whole inquiry is conducted with much philosophical acumen. Hypothetical, of course, part of it must be: but how different are the hypotheses of the present from those of former times, when science was a sort of poetry, and dealt in abstractions and inventions!” * * * * * ESSAY ON CLASSIFICATION.