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which the ancients perceived between the moon and the tides, to that view of the case which is now afforded us by the doctrine of universal gravitation, acting between the sun, the moon, the solid earth, and the waters on its surface, we arrive at a far more generalized sort of relation; we refer the phenomenon not only to an invariable sequence, but assert the dependence of a particular effect upon a very general principle: we mean that the tides are a particular case of the universal law of gravitation.

The same may be said of our assertion, that “electric discharge is the cause of lightning and thunder;" that “ volcanic action is the cause of earthquakes ;" in a vast number of such instances, what we really mean will be found to include a reference to a more general fact, law, or principle. We mean, that lightning and thunder are a particular case of electric discharge; or are reducible to that greater principle: an earthquake a particular case of the effect of volcanic force: that is, in all such cases, we use the term cause to include the idea that the one phenomenon (which we call the effect) is a particular kind of the other (or cause) which is the more general fact: the relation is not only that of sequence, but also that of a species referred to its genus.

When the suspension of water in the pump was first observed it was ascribed to a cause called “ suction,” and in the then state of knowledge, it was not only natural, but inductively correct, to ascribe so singular an effect to a peculiar cause. It .

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was apparently a case“ sui generis :" the effect was, perhaps, soon seen to be of the same kind as the suspension of a stone by contact with the under side of a wet leather; there was then one step taken in the process of generalization, by referring both to one common cause, still named suction. Further, the discovery of Torricelli referred the former case to the pressure of the atmosphere; and this was soon seen to include the explanation of the latter and all other analogous phenomena. And, finally, this was reduced under the still more comprehensive law of universal gravitation. Thus the gross and heavy matter of the earth (as the scholastic systems considered it), and the pure and æthereal substance of the atmosphere, were brought under the common dominion of one simple and general law. Thus the study of causes proceeds from the first admission of a peculiar phenomenon as a thing “sui generis,” until it is shown to be reducible to a species of some more extensive established “ genus.”

When a narrow beam of solar light passes the sides of a wire, or the edges of an aperture, certain coloured stripes or fringes are exhibited on a screen placed at some distance. These facts were at first ascribed (very justly) to a peculiar property of light; which was termed “inflexion," or sometimes “ diffraction.”

The colours of soap bubbles, or those between two glasses pressed hard together, were ascribed by Newton to another peculiar property of light, by

virtue of which it has different characters alternately, at successive intervals, along the length of the ray. These alternations of character were termed 6 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

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* See my Treatise on Optics, p. 125, Oxford, 1833,
+ See above, p. 53.

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.

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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

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