{The Newtonian Law of Gravity}

The Nebular Theory of Laplace has lately received far more confirmation than it needed, at the hands of the philosopher, Compte. These two have thus together shown – not, to be sure, that Matter at any period actually existed as described, in a state of nebular diffusion, but that, admitting it so to have existed throughout the space and much beyond the space now occupied by our solar system, and to have commenced a movement towards a centre – it must gradually have assumed the various forms and motions which are now seen, in that system, to obtain. A demonstration such as this – a dynamical and mathematical demonstration, as far as demonstration can be – unquestionable and unquestioned – unless, indeed, by that unprofitable and disreputable tribe, the professional questioners – the mere madmen who deny the Newtonian law of Gravity on which the results of the French mathematicians are based – a demonstration, I say, such as this, would to most intellects be conclusive – and I confess that it is so to mine – of the validity of the nebular hypothesis upon which the demonstration depends.

That the demonstration does not prove the hypothesis, according to the common understanding of the word ›proof,‹ I admit, of course. To show that certain existing results – that certain established facts – may be, even mathematically, accounted for by the assumption of a certain hypothesis, is by no means to establish the hypothesis itself. In other words: – to show that, certain data being given, a certain existing result might, or even must, have ensued, will fail to prove that this result did ensue, from the data, until such time as it shall be also shown that there are, and can be, no other data from which the result in question might equally have ensued. But, in the case now discussed, although all must admit the deficiency of what we are in the habit of terming ›proof,‹ still there are many intellects, and those of the loftiest order, to which no proof could bring one iota of additional conviction. Without going into details which might impinge upon the Cloud-Land of Metaphysics, I may as well here observe that the force of conviction, in cases such as this, will always, with the right-thinking, be proportional to the amount of complexity intervening between the hypothesis and the result. To be less abstract: – The greatness of the complexity found existing among cosmical conditions, by rendering great in the same proportion the difficulty of accounting for all these conditions at once, strengthens, also in the same proportion, our faith in that hypothesis which does, in such manner, satisfactorily account for them: – and as no complexity can well be conceived greater than that of the astronomical conditions, so no conviction can be stronger – to my mind at least – than that with which I am impressed by an hypothesis that not only reconciles these conditions, with mathematical accuracy, and reduces them into a consistent and intelligible whole, but is, at the same time, the sole hypothesis by means of which the human intellect has been ever enabled to account for them at all.

A most unfounded opinion has been latterly current in gossiping and even in scientific circles – the opinion that the so-called Nebular Cosmogony has been overthrown. This fancy has arisen from the report of late observations made, among what hitherto have been termed the ›nebulæ,‹ through the large telescope of Cincinnati, and the world-renowned instrument of Lord Rosse. Certain spots in the firmament which presented, even to the most powerful of the old telescopes, the appearance of nebulosity, or haze, had been regarded for a long time as confirming the theory of Laplace. They were looked upon as stars in that very process of condensation which I have been attempting to describe. Thus it was supposed that we ›had ocular evidence‹ – an evidence, by the way, which has always been found very questionable – of the truth of the hypothesis; and, although certain telescopic improvements, every now and then, enabled us to perceive that a spot, here and there, which we had been classing among the nebulæ, was, in fact, but a cluster of stars deriving its nebular character only from its immensity of distance – still it was thought that no doubt could exist as to the actual nebulosity of numerous other masses, the strong-holds of the nebulists, bidding defiance to every effort at segregation. Of these latter the most interesting was the great ›nebulæ‹ in the constellation Orion: – but this, with innumerable other miscalled ›nebulæ,‹ when viewed through the magnificent modern telescopes, has become resolved into a simple collection of stars. Now this fact has been very generally understood as conclusive against the Nebular Hypothesis of Laplace; and, on announcement of the discoveries in question, the most enthusiastic defender and most eloquent popularizer of the theory. Dr. Nichol, went so far as to ›admit the necessity of abandoning‹ an idea which had formed the material of his most praiseworthy book.⁶

Many of my readers will no doubt be inclined to say that the result of these new investigations has at least a strong tendency to overthrow the hypothesis; while some of them, more thoughtful, will suggest that, although the theory is by no means disproved through the segregation of the particular ›nebulæ‹ alluded to, still a failure to segregate them, with such telescopes, might well have been understood as a triumphant corroboration of the theory: – and this latter class will be surprised, perhaps, to hear me say that even with them I disagree. If the propositions of this Discourse have been comprehended, it will be seen that, in my view, a failure to segregate the ›nebulæ‹ would have tended to the refutation, rather than to the confirmation, of the Nebular Hypothesis.

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⁶ »Views of the Architecture of the Heavens.« A letter, purporting to be from Dr. Nichol to a friend in America, went the rounds of our newspapers, about two years ago, I think, admitting ›the necessity‹ to which I refer. In a subsequent Lecture, however, Dr. N. appears in some manner to have gotten the better of the necessity, and does not quite renounce the theory, although he seems to wish that he could sneer at it as ›a purely hypothetical one.‹ What else was the Law of Gravity before the Maskelyne experiments? and who questioned the Law of Gravity, even then?