Various - Blackwood's Edinburgh Magazine, No. CCCXXXVI. October, 1843. Vol. LIV.

"With respect to the general law of causation, it does appear that there must have been a time when the universal prevalence of that law throughout nature could not have been affirmed in the same confident and unqualified manner as at present. There was a time when many of the phenomena of nature must have appeared altogether capricious and irregular, not governed by any laws, nor steadily consequent upon any causes. Such phenomena, indeed, were commonly, in that early stage of human knowledge, ascribed to the direct intervention of the will of some supernatural being, and therefore still to a cause. This shows the strong tendency of the human mind to ascribe every phenomenon to some cause or other; but it shows also that experience had not, at that time, pointed out any regular order in the occurrence of those particular phenomena, nor proved them to be, as we now know that they are, dependent upon prior phenomena as their proximate causes. There have been sects of philosophers who have admitted what they termed Chance as one of the agents in the order of nature by which certain classes of events were entirely regulated; which could only mean that those events did not occur in any fixed order, or depend upon uniform laws of causation. * * *

"The progress of experience, therefore, has dissipated the doubt which must have rested upon the universality of the law of causation, while there were phenomena which seemed to be sui generis ; not subject to the same laws with any other class of phenomena, and not as yet ascertained to have peculiar laws of their own. This great generalisation, however, might reasonably have been, as it in fact was by all great thinkers, acted upon as a probability of the highest order, before there were sufficient grounds for receiving it as a certainty. For, whatever has been found true in innumerable instances, and never found to be false after due examination in any, we are safe in acting upon as universal provisionally, until an undoubted exception appears; provided the nature of the case be such that a real exception could scarcely have escaped our notice. When every phenomenon that we ever knew sufficiently well to be able to answer the question, had a cause on which it was invariably consequent, it was more rational to suppose that our inability to assign the causes of other phenomena arose from our ignorance, than that there were phenomena which were uncaused, and which happened accidentally to be exactly those which we had hitherto had no sufficient opportunity of studying."—Vol. II. p. 108.

Hypotheses. —Mr Mill's observations on the use of hypotheses in scientific investigation, except that they are characterized by his peculiar distinctness and accuracy of thought, do not differ from the views generally entertained by writers on the subject. We are induced to refer to the topic, to point out what seems to us a harsh measure dealt out to the undulatory theory of light—harsh when compared with the reception given to a theory of Laplace, having for its object to account for the origin of the planetary system.

We had occasion to quote a passage from Mr Mill, in which he remarks that the majority of scientific men seem not yet to have completely got over the difficulty of conceiving matter to act (contrary to the old maxim) where it is not; "for though," he says, "they have at last learned to conceive the sun attracting the earth without any intervening fluid, they cannot yet conceive the sun illuminating the earth without some such medium." But it is not only this difficulty (which doubtless, however, is felt) of conceiving the sun illuminating the earth without any medium by which to communicate its influence, which leads to the construction of the hypothesis, either of an undulating ether, or of emitted particles. The analogy of the other senses conducts us almost irresistibly to the imagination of some such medium. The nerves of sense are, apparently, in all cases that we can satisfactorily investigate, affected by contact, by impulse. The nerve of sight itself, we know, when touched or pressed upon, gives out the sensation of light. These reasons, in the first place, conduct us to the supposition of some medium, having immediate communication with the eye; which medium, though we are far from saying that its existence is established, is rendered probable by the explanation it affords of optical phenomena. At the same time it is evident that the hypothesis of an undulating ether, assumes a fluid or some medium, the existence of which cannot be directly ascertained. Thus stands the hypothesis of a luminiferous ether—in what must be allowed to be a very unsatisfactory condition. But a condition, we think, very superior to the astronomical speculation of Laplace, which Mr Mill, after scrutinizing the preceding hypothesis with the utmost strictness, is disposed to treat with singular indulgence.

"The speculation is," we may as well quote throughout Mr Mill's words, "that the atmosphere of the sun originally extended to the present limits of the solar system: from which, by the process of cooling, it has contracted to its present dimensions; and since, by the general principles of mechanics, the rotation of the sun and its accompanying atmosphere must increase as rapidly as its volume diminishes, the increased centrifugal force generated by the more rapid rotation, overbalancing the action of gravitation, would cause the sun to abandon successive rings of vaporous matter, which are supposed to have condensed by cooling, and to have become our planets.

"There is in this theory," Mr Mill proceeds, "no unknown substance introduced upon supposition, nor any unknown property or law ascribed to a known substance. The known laws of matter authorize us to suppose, that a body which is constantly giving out so large an amount of heat as the sun is, must be progressively cooling, and that by the process of cooling it must contract; if, therefore, we endeavour, from the present state of that luminary, to infer its state in a time long past, we must necessarily suppose that its atmosphere extended much further than at present, and we are entitled to suppose that it extended as far as we can trace those effects which it would naturally leave behind it on retiring; and such the planets are. These suppositions being made, it follows from known laws that successive zones of the solar atmosphere would be abandoned; that these would continue to revolve round the sun with the same velocity as when they formed part of his substance, and that they would cool down, long before the sun himself, to any given temperature, and consequently to that at which the greater part of the vaporous matter of which they consisted would become liquid or solid. The known law of gravitation would then cause them to agglomerate in masses, which would assume the shape our planets actually exhibit; would acquire, each round its own axis, a rotatory movement; and would in that state revolve, as the planets actually do, about the sun, in the same direction with the sun's rotation, but with less velocity, and each of them in the same periodic time which the sun's rotation occupied when his atmosphere extended to that point; and this also M. Comte has, by the necessary calculations, ascertained to be true, within certain small limits of error. There is thus in Laplace's theory nothing hypothetical; it is an example of legitimate reasoning from a present effect to its past cause, according to the known laws of that case; it assumes nothing more than that objects which really exist, obey the laws which are known to be obeyed by all terrestrial objects resembling them."—Vol. II. p. 27.

Now, it seems to us that there is quite as much of hypothesis in this speculation of Laplace as in the undulatory theory of light. This atmosphere of the sun extending to the utmost limits of our planetary system! What proof have we that it ever existed? what possible grounds have we for believing, what motive even for imagining such a thing, but the very same description of proof given and rejected for the existence of a luminiferous ether—namely, that it enables us to explain certain events supposed to result from it? Nor is the thing here imagined any the less a novelty, because it bears the old name of an atmosphere. An atmosphere containing in itself all the various materials which compose our earth, and whatever else may enter into the composition of the other planets, is as violent a supposition as an ether, not perceptible to the senses except by its influence on the nerves of sight. And this cooling down of the sun! What fact in our experience enables us to advance such a supposition? We might as well say that the sun was getting hotter every year, or harder or softer, or larger or smaller. Surely Mr Mill could not have been serious when he says, that "the known laws of matter authorize us to suppose, that a body which is constantly giving out so large an amount of heat as the sun is, must be progressively cooling"—knowing, as we do, as little how the sun occasions heat as how it produces light. Neither can it be contended that because no absolutely new substance, or new property of matter, is introduced, but a fantastic conception is framed out of known substances and known properties, that therefore there is less of rash conjecture in the supposition. In fine, it must be felt by every one who reads the account of this speculation of Laplace, that the only evidence which produces the least effect upon his mind, is the corroboration which it receives from the calculations of the mathematician—a species of proof which Mr Mill himself would not estimate very highly.