virtue of which it has different characters alternately, at successive intervals, along the length of the ray. These alternations of character were termed "fits." Some other phenomena of an analogous nature were, in like manner, set down as due to properties of light, each entirely " sui generis." At length, however, it was shown by Dr. Young, that they could all be reduced to a common principle (or genus), named "interference;" (before alluded tot;) and which supposed two rays of light capable of either conspiring in their effects, or neutralizing each other, as they coincided at similar or dissimilar points or periods, at alternate minute distances along their lengths*: and such a property was accounted for by the supposition of light consisting in waves propagated through an infinitely rare and elastic medium. When a ray was transmitted through certain media, or reflected at a certain angle, it is found to have different properties along its opposite sides. This was a distinct fact, and was referred to a peculiar cause, termed "polarization." When polarized light is received through a tourmaline (or other analyzing apparatus) in a particular position, it wholly disappears. But if a plate of certain crystals be interposed, a portion of the light, or certain particular colours, are restored. This, again, was considered at first a distinct property, and was * See my Treatise on Optics, p. 125, Oxford, 1833, called "depolarization." It has been, however, since perfectly explained by the general principle of " interference," if combined with an explanation of polarization, which is also easily supplied by referring it to the same general theory of waves. The cooling of the earth at night is the cause of the deposition of dew on those substances which cool fastest, from the condensation of the suspended moisture of the atmosphere upon them. Here the cooling or radiation of the earth, considered merely as any other heated body, is a greater or more general principle; such also is the more rapid cooling of fibrous substances; and such, again, is the suspension of invisible vapour in the atmosphere, and its condensation on a certain lowering of temperature. We are thus enabled to refer the particular instance of the dew on the grass (and not on the gravel) to a union of these more general principles. Here, then, we have a combination of causes, involving a far more satisfactory and intimate kind of relation. True and Hypothetical Causes. THIS will be the proper place to notice an important distinction, often not clearly kept in view. We may refer to a cause, or general principle, which is likewise independently established by other experimental evidence as something actually existing in nature apart from the particular cases we were at first considering. But also, in other cases, we may find some mere gratuitous supposition or hypothesis which supplies a connecting principle or general representation of the facts, though in itself purely fictitious. Such a principle is sometimes termed a "cause," as well as the former. And such cases have been compared to the discovery of a key to a cipher, obtained merely by finding, from repeated trials, that, in point of fact, it does supply an interpretation. And some writers have contended that this is all we want in inductive philosophy, and would restrict the study of causes solely to this object. The illustration, however, at best appears to me to convey but a miserable idea of the investigation of physical causes; and if adopted exclusively, cuts off the most valuable and satisfactory subjects of physical inquiry. Newton (at the commencement of the third book of his Principia,) has laid down certain rules for the study of physical causes; the chief of these, and a rule supported on the soundest considerations, points out two conditions as the characteristics of legitimate physical causes, viz., that they be such as are "true," and "sufficient to explain the phenomena." Now an hypothetical cause (or the mere key to the cipher,) fulfils but one of these conditions. Nevertheless such hypotheses may be eminently useful, especially when we can attain to nothing better. With the reference to such principles, we are often compelled to be satisfied. They afford a good representation of the facts, though they have no other existence than as so representing them. But on all grounds, it is a far higher and more satisfactory process if we can arrive at such a cause as fulfils both Newton's conditions; such as not only affords a key to the phenomena, but is also "true," or is proved to be a real principle existing in nature by other and independent considerations. Examples in Physics. 1. THE varied phenomena of electro-magnetism are all explained by the supposition of a system of currents in directions transverse to the length of the needle. But these currents are not proved to exist by any independent evidence. This, then, is an instance of an hypothetical cause, which yet explains the phenomena. 2. On the other hand, the air is a medium really and independently known to exist; and pulsations are real mechanical effects produced in it. The phenomena of sound are thus referred to a “real cause," ," which perfectly "explains" them. 3. The explanation of dew before mentioned is also an example of combination of "true" causes. 4. We before alluded to large classes of optical facts which are reducible to the principle of " interference." That rays of light actually possess some inherent property by virtue of which they can so interfere, has been shown by independent experiment, by Arago and Fresnel. Interference, then, is a real effect; and these classes of facts are explained by a "true cause." The fact of interference itself is again explained perfectly by the theory of waves, propagated in an infinitely rare and elastic medium; also a vast number of other optical facts which have no connexion with interference, and others which have, are all capable of exact explanation by this hypothesis of waves; which, when modified by some peculiar considerations, seems likely to afford a clue to nearly all the most complex phenomena of light. Yet we have no independent proof of the existence of an æther, or the propagation of waves in it. It therefore remains at present an hypothetical cause. 5. Gravitation, or the tendency of matter to fall together with a force proportional directly to the mass and inversely to the square of the distance, is a real thing; we find it independently and experimentally in the attractions which take place within the reach of our investigations; we also find that the extension of the same cause perfectly explains all the movements of the planetary system. Examples in Geology. 6. THE phenomena presented by the actual state and structure of the earth's surface are such as both |