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

admit and call for inquiry into the nature of the causes to which they can be referred.

The pursuit of this inquiry soon discloses the evidence and monuments of successive changes which have occurred in the state of the earth's surface. In the attempt to trace these to their causes, sound inductive geology recognises, of course, the same principle of referring to those which are both true, and sufficient to explain the phenomena. We cannot find true causes except in such as are really proved to exist, and found by experience to be in operation. The action of the waters on the land, (whether the continued power of the rivers and ocean, or the occasional force of inundations and torrents,) the subterranean force of earthquakes, and the external operation of volcanoes; the contractions and expansions which must accompany changes in the temperature of any considerable thickness of the earth's crust: these and the like are the real causes to which the sound geologist refers.

The accumulation of soil at the bottom of the waters, the imbedding of animal and vegetable remains in those depositions, the elevation of portions of land out of the sea, are operations really and continually going on. When, therefore, fossil remains of organized beings are found imbedded in rocks bearing also the marks of a similar mode of deposition, we refer to such operations as those just mentioned as true causes to explain the phenomena. And numerous series and successions of such depo

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