LECTURE I.

INTRODUCTORY.

Classification of Recent Advances in Physical Science. General Statement of the Objects of Physics. Time and Space. Matter, Position, Motion, and Force. Digression upon à priori reasoning. Instances of modern or revived fallacies-Uniformity of Earth's Rotation, Stability of Solar System, Heat developed the equivalent of work spent in compressing a gas, Causa aquat effectum. Gilbert the true originator of Experimental Science. Test of the reality of Matter-fails when applied to Force--not when applied to Energy. Conservation, Transformation, and Dissipation of Energy. Ignorance and Incapacity alike of Spiritualists and Materialists.

IN considering what may be designated as 'Recent Advances in Physical Science,' it is well to remember that many things which have become almost popularly known within the last twenty-five years are much older in the minds and writings of the foremost scientific men. We cannot, however, treat them intelligibly without reference, sometimes pretty full, to what was known even earlier still: so that you must not be surprised if I have a good deal to say of Davy and Rumford, and even of Newton.

I shall, for the sake of clearness, attempt roughly to classify these recent advances under five well-marked heads; but I shall do so very briefly, deferring explanation even of new scientific terms till I have to treat each of these heads in detail.

First and foremost, advances connected with the

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modern notion of Energy. Just as Gold, Lead, Oxygen, etc., are different kinds of Matter, so Sound, Light, Heat, etc., are now ranked as different forms of Energy, which, as we shall presently see, has been shown to have as much claim to objective reality as matter has. This grand idea enables us to co-ordinate all the parts, however apparently diverse, of the enormous subject of Natural Philosophy. It has not only thus enabled us to exhibit the science in a complete and connected form, but it has also, specially by the application of the laws of Thermo-dynamics (to which a large part of this course will be devoted), enabled us to find those points where rapid advance was most easily to be secured.

Secondly. The advances which have arisen, more or less directly, from the requirements felt in practical applications. To take but a single instance: think of the immense improvements in instruments for the measurement of electric charges and electric currents, such as electrometers and galvanometers, which have been effected because called for by the recent extensions of submarine telegraphy. It is not too much to say that the instruments now employed, and which were primarily devised for practical telegraphic purposes, are hundreds of times more sensitive, as well as more exact, and therefore more useful for purely scientific purposes, than the best of those which were in use thirty years ago. Thus it is that a development of science, in a practical direction, leads to the construction of instruments which have, as it were, a reflex action on the development of the pure science itself.

Thirdly. Those which arise from the assistance rendered to one another by pure sciences, such as astronomy, chemistry, and physiology, where, in fact, the

improvement of one branch has led, almost immediately, to important extensions of other branches. Under this head we may also include those very great advances which are due to improvements in our mathematical methods.

Fourthly. What may be called casual discoveries, though they are often of very great importance; such as, for instance, the discovery of fluorescence, with its manifold consequences, and the invention of the processes of photography. Such discoveries, instead of being, as in old days, wondered at and left isolated, are now at once attacked on all sides by numberless enthusiastic experimenters.

Fifthly. There is another class, very numerous but more difficult to exactly describe. As a single example of this class, I may mention the modern statistical methods of treating certain problems of physical science, especially those connected with the movements of particles of gases and liquids, to which I shall advert at considerable length in the course of these lectures.

I have now to consider how I should best commence the analysis of these various heads; and I think the proper method will be first to sketch the subject as if from a distance-to point out a few of the principal peaks which we have to ascend, and of the more formidable abysses which we have to avoid; striving all the while to introduce as early as possible some of those new technical terms which are absolutely indispensable to accuracy and definiteness, and which, therefore, can not be too soon mastered.

Natural Philosophy, as now regarded, treats generally of the physical universe, and deals fearlessly alike with quantities too great to be distinctly conceived, and with

quantities almost infinitely too small to be perceived even with the most powerful microscopes; such as, for instance, distances through which the light of stars or nebulæ, though moving at the rate of about 186,000 miles per second, takes many years to travel; or the size of the particles of water, whose number in a single drop may, as we have reason to believe, amount to somewhere about

1026, or 100,000,000,000,000,000,000,000,000.

Yet we successfully inquire not only into the composition of the atmospheres of these distant stars, but into the number and properties of these water-particles, nay, even into the laws by which they act upon one another.

The fundamental notions which occur to us when we commence the study of physical science are those of Time and Space. A measure of time may be obtained by physical methods, as in fact is done incidentally in Newton's First Law of Motion, wherein he asserts that a mass left to itself moves uniformly. That is, equal times are the times in which such a mass describes equal spaces. Of space, we can ascertain by observation the properties. But we cannot inquire into the actual nature of either space or time, except in the way of a purely metaphysical, and therefore of necessity absolutely barren, speculation. We have, however, mathematical methods specially adapted to the treatment of these two abstract ideas; Algebra, which has been called (by Sir W. R. Hamilton) the science of pure time; and Geometry, which may be designated the science of pure space.

The common measurement of time primarily depends upon the rotation of the earth about its axis. This, however, as will be seen when we advance a little

further, is by no means a uniform quantity, and therefore ultimately the measurement of time must be based upon some motion depending on a physical property of matter which we have every experimental reason for believing to be unchangeable by time, and invariable throughout the universe. Probably such an ultimate standard for the measurement of time will be found in one of the periods of vibration of the molecules of a heated gas, such as hydrogen, under given conditions.

The properties of space, involving (we know not why) the essential element of three dimensions, have recently been subjected to a careful scrutiny by mathematicians of the highest order, such as Riemann and Helmholtz;1 and the result of their inquiries leaves it as yet undecided whether space may or may not have precisely the same properties throughout the universe. To obtain an idea of what is meant by such a statement, consider that in crumpling a leaf of paper, which may be taken as representing space of two dimensions, we may have some portions of it plane, and other portions more or less cylindrically or conically curved. But an inhabitant of such a sheet, though living in space of two dimensions only, and therefore, we might say beforehand, incapable of appretiating the third dimension, would certainly feel some difference of sensations in passing from portions of his space which were less, to other portions which were more, curved. So it is possible that in the rapid march of the solar system through space, we may be gradually passing to regions in which space has not precisely the same properties as we find here-where it may have something in three dimensions analogous to curvature in two 1 See Helmholtz' paper in Mind, No. III. 1876.