would have been absorbed in Natural Philosophy. But there are more than sixty different kinds of substance, as gold, silver, charcoal, sulphur, and others, which, at least according to present knowledge, are believed to be quite distinct. Now all the Laws of Natural Philosophy hold with regard to each of these kinds of substance separately, so that no new science is introduced as yet. Moreover observation and record of the special properties of each kind of substance would be included in what is popularly known as Natural History. But it is found that when two or more of these different kinds of substance are brought together under certain circumstances, then special phenomena are seen. Thus, for example, fine sand and powdered soda exposed to heat and melted together become glass, which differs from each of its components as to its distinctive properties. Again, the metal sodium is poisonous, if swallowed, and the gas chlorine if breathed; if these are brought together they explode and burst into a flame, and the result of the combustion is common salt, which is very wholesome. Chemistry then treats of all the phenomena which are connected with the combination of two or more kinds of substance to form a new body, or with the separation of any body into the simple kinds of substance of which it may consist. The science had its origin in the attempts made by enthusiasts to convert the more common metals into gold the most valuable: these men received the name of alchemists.

47. Next we have to consider the sciences which involve the idea of Life. The bodies of men and of animals, and the vegetable structures, consist of various remarkable collections of tubes and cavities, in which fluids circulate and produce constant change. In addition to the laws which prevail and the forces which act in Natural Philosophy and Chemistry, others of a peculiar kind here present themselves; instead of the permanence which belongs more or less to the objects of the two former divisions of knowledge, we have here the changes involved in birth, growth, and decay.

48. In the last place we have sciences which relate to the mind itself, as Logic and Metaphysics. These have

been cultivated from the origin of civilization, and though they have passed through various fluctuations of influence have never lost their charm. "Even in ages the most devoted to material interests, some portion of the current of thought has been reflected inwards, and the desire to comprehend that by which all else is comprehended has only been baffled in order to be renewed." In this division we may place various studies which bear, at least indirectly, on the mind-as that of Languages which has long been held of great value as a training, History which teaches by example the lessons of duty and prudence, and Moral Philosophy which gathers these lessons into a system, and seeks to enforce them by adequate sanctions.

49. It is easy to see that the rough division which we have given of the branches of Human Knowledge is open to the objection of a failure in distinctness; some sciences may claim to appear under more than one of the five classes. Thus we make Mathematics a distinct class, and yet it must be allowed that this science enters largely into all the elaborate works on Natural Philosophy. The aid of Mathematics is absolutely necessary in order to develop fully the principles which are discovered in operation throughout nature; and not unfrequently the wish to penetrate further into the constitution of the earth and the heavens has led men to the construction of new methods in Mathematics. The sciences which we include under the title Natural Philosophy have sometimes been called Mixed Mathematics, while the title Pure Mathematics has been adopted as more strictly appropriate to the first of our five classes. Up to the present time Chemistry has not been annexed to Mathematics; but eminent men, among whom Faraday may be named, have pointed with satisfaction to that as the destiny of their science.

50. Again, some of the subjects which we have included under Natural Philosophy are closely connected with Chemistry. Thus, many of the changes which Chemistry investigates are produced by the agency of Heat; so that this subject belongs both to the third and to the second class of our arrangement. A similar remark holds with respect to Electricity and the kindred sciences. Again,

Photography claims the attention of the Natural Philosopher as falling under the head of Light, and is at the same time of great interest to the Chemist by reason of the sensitive materials on which the impressions of the sun's rays are received.

51. Some sciences may appear to be without a place in our classes. Thus for instance, Geology and Mineralogy are not very happily put under Chemistry, though this seems to be the least unsuitable station for them. Moreover subjects which in many respects it would be very convenient to associate become separated by our arrangement. Thus all the components which make up the tolerably well defined aggregate called Natural History are broken up and distributed over the third and fourth classes. The arrangement in five classes does however agree reasonably well with the schemes proposed by some philosophers who have paid special attention to the classification of knowledge. The late Dr Whewell, whose great and varied attainments rendered him specially qualified to deal with such a matter, gives implicitly in his Philosophy of the Inductive Sciences a scheme of classification. The subjects which are included in our first three classes are arranged by him in the following manner: Pure Sciences; Mechanical Sciences; Secondary Mechanical Sciences, namely, Sound, Light, and Heat; Mechanico-Chemical Sciences, namely, Magnetism and Electricity; and Chemistry. Besides these he treats of the Natural History sciences under various titles.

52. With the Sciences are connected Arts in which the lessons of theory are applied to purposes of utility and ornament. Thus Engineering, Architecture, and Navigation are all Arts connected with the Sciences of Natural Philosophy. Dr Whewell was fond of calling Art the lovely mother, and Science the daughter of far loftier and serener beauty; there is no doubt that in the main this is historically correct. Some simple processes of engineering must have preceded the science of mechanics, and some rough comparisons of size and figure must have been made before the truths of geometry were arranged in a system. Not only the ruder arts relating to food and clothing which

sustain our lives, but even those of a more refined character such as music, painting, and sculpture, which adorn them, must have made some progress before the sciences arose to explain the principles on which such arts depend. Still in recent times science has more than repaid all that she ever borrowed from art; such important applications as the steam-engine, photography, and the electric telegraph were, we know, derived almost exclusively from science.

53. Controversy has been often maintained with respect to the subjects which are most valuable for the education of youth, and to the order in which they should be presented for study. The older theory in England was that the languages of Greece and Rome were the best instruments for cultivating the powers of the mind, and that the literature preserved in these languages was the most valuable treasury that could be found of history and poetry and mental science. In one University Mathematics have long been highly valued as a discipline for maturer years, and recently they have gained some position in the course of school instruction. Other studies however in the present day urge their claims to attention; modern languages are cultivated principally on the ground that they give access to rich stores of information; Chemistry and the Natural Sciences demand and receive considerable regard, on account of their important practical applications. Perhaps the acquisition of knowledge is appreciated more highly now than in former days, and the mere training of the powers of the mind less exclusively considered. However we must not forget that many able and enthusiastic men attribute as much merit to the favourite modern studies as our forefathers assumed to belong to the Classical Languages and Mathematics. Thus Dr Arnott says, "Reverting to the importance of Natural Philosophy as a general study, it may be remarked that there is no occupation which so much strengthens and quickens the judgment. This praise has often been awarded to the Mathematics, although a knowledge of abstract Mathematics existed with all the absurdities of the dark ages; but a familiarity with Natural Philosophy, which includes fundamental Mathematics, and gives tangible and pleasing illustrations of the abstract truths, seems incompatible with the admission of any gross absurdity."

T. P.

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III. NATURAL PHILOSOPHY.

54. It is the design of the present work to consider an important part of the second of the five classes into which we have divided knowledge in Art. 43; and it will be convenient here to offer a few preliminary remarks which will bring the more important facts into view. The reader will probably not fully comprehend at first all that this Chapter contains, but he can hardly fail to obtain from it some general notions which will be of assistance to him as he proceeds through the rest of the work. The advance in knowledge which an individual student obtains by the devotion of time and attention to a science is similar in character to the progress which the science itself makes in the course of ages; the student can trace his way backwards to a clearer view of the first principles, and forwards to more extensive developments and applications.

55. In such a sketch as we are now about to give, the reader enters into possession of knowledge which has been accumulated by centuries of thought and labour. The tendency of this long series of investigations has been to produce a firm conviction that order and law prevail throughout nature; and that often apparently contradictory phenomena result from the operation of one general principle. Thus, for instance, most things fall to the ground when unsupported, while a few, like smoke, or bubbles, or balloons rise. Hence it might seem that there is a difference in the structure of bodies or in the substance of which they consist, in virtue of which some will descend and some will ascend, when set free. The notion is embodied in the well known witticism respecting a man who gained an eminent position by a discreet sobriety of manner, and then lost respect by his want of official decorum: "contrary to the laws of physics he rose by gravity and sank by levity." But we know now that all the bodies with which we are concerned are really heavy, though some are much heavier,