Now the subjects of heat and light are obviously of the utmost importance to the chemical philosopher, and they are very extensive, and intricate and difficult to treat of, inasmuch as the writer is necessarily upon the confines of chemical and mechanical philosophy, and should be expert in both. When, therefore, elementary works on chemistry are so written and arranged as to serve as text-books for lectures, and indexes of reference to more accurate information, we can make due allowance for brevity; but when the subject is intended to be formally and completely developed to the student, independent of other ocular and oral aids, much more extensive description and detailed explanation is required, than is to be found either in our author’s “Elements,” or in any other analogous condensation of chemistry. Dr. Henry understands the requisite mode of conveying information in these cases better than most writers; and when he takes pains, and speaks for himself, has the talent of being brief, and at the same time minute, deep, and clear. Dr. Ure, as his dictionary shows, is an eminent example of such a writer—_he_ of course is neglected, where, as with our author, Dr. Thomson is in the ascendant; but the article caloric, in his dictionary, will at once explain and illustrate our meaning, and would furnish an admirable foundation for a detailed essay or treatise upon the subject. So extensive, indeed, are the precincts of chemistry now becoming, that either our _systems_ must become very voluminous, or we must adopt the plan, which to us appears preferable, of distinct treatises upon different branches of the science. Thus, a separate work on heat and light; another on electricity and magnetism; another on attraction and the theory of combination; a fourth on the constitution and properties of the unmetallic elementary bodies; a fifth on the metals and their compounds; a sixth on vegetable, and a seventh on animal chemistry and physiology; an eighth on the chemistry of the arts; and lastly, a treatise on chemical manipulation in general, would include all that appears essentially requisite; and as no one is supposed to be equally well versed in all branches of the science, or in all details of the art, an opportunity of selection would thus be afforded, so that each writer might choose that particular department which he is most accurately acquainted with, or which has formed his favourite study. Mr. Faraday has already, as may be said, led the way in such a plan, by the publication of his _Chemical [p062] Manipulation_, a work hitherto exceedingly wanted in the laboratory, equally useful to the proficient and to the student, and eminently creditable to the industry and skill of the author, and to the school whence it emanates. We shall of course take an early opportunity of introducing this book in a more formal way to the attention of our chemical readers.