rays. Such an hypothesis then is inconsistent with the main body of our knowledge concerning gases.

Provided that there be no clear and absolute conflict with known laws of nature, there is nothing so improbable or apparently inconceivable that it may not be rendered highly probable, or even approximately certain, by a sufficient number of concordances. In fact the two best founded and most conspicuously successful theories in the whole range of physical science involve the most absurd suppositions. Gravity is a force which appears to act between bodies through vacuous space; it is in positive contradiction to the old dictum that nothing could act but through some intervening medium or substance. It is even more puzzling that the force acts in perfect indifference to all intervening obstacles. Light in spite of its extreme velocity, shows much respect to matter, for it is almost instantaneously stopped by opaque substances, and to a considerable extent absorbed and deflected by transparent ones. But to gravity all media are, as it were, absolutely transparent, nay non-existent; and two particles at opposite points of the earth affect each other exactly as if the globe were not between. To complete the apparent impossibility, the action is, so far as we can observe, absolutely instantaneous, so that every particle of the universe is at every moment in separate cognizance, as it were, of the relative position of every other particle throughout the universe at that same moment of absolute time. Compared with such incomprehensible conditions, the theory of vortices deals with common-place realities. Newton's celebrated saying, hypotheses non fingo, bears the appearance of pure irony; and it was not without apparent grounds that Leibnitz and the greatest continental philosophers charged Newton with re-introducing occult powers and qualities.

The undulatory theory of light presents almost equal

difficulties of conception. We are asked by physical philosophers to give up all our ordinary prepossessions, and believe that the interstellar space which seemed so empty is not empty at all, but filled with something immensely more solid and elastic than steel. As Dr. Young himself remarked, the luminiferous ether, pervading all space, and penetrating almost all substances, is not only highly elastic, but absolutely solid!!!' Sir John Herschel has calculated the amount of force which may be supposed, according to the undulatory theory of light, to be exerted at each point in space, and finds it to be 1,148,000,000,000 times the elastic force of ordinary air at the earth's surface, so that the pressure of the ether upon a square inch of surface must be about 17,000,000,000,000, or seventeen billions of pounds. Yet we live and move without appreciable resistance through this medium, indefinitely harder and more elastic than adamant. All our ordinary notions must be laid aside in contemplating such an hypothesis; yet they are no more than the observed phenomena of light and heat force us to accept. We cannot deny even the strange suggestion of Dr. Young, that there may be independent worlds, some possibly existing in different parts of space, but others perhaps pervading each other unseen and unknown in the same space. For if we are bound to admit the conception of this adamantine firmament, it is equally easy to admit a plurality of such. We see, then, that mere difficulties of conception must not in the least discredit a theory which otherwise agrees with facts, and we must only reject hypotheses which are inconceivable in the sense of breaking distinctly the primary laws of thought and nature.

f Young's 'Works,' vol. i. p. 415.

6

Familiar Lectures on Scientific Subjects,' p. 282.

h Young's Works,' vol. i. p. 417.

The Third Requisite-Conformity with Facts.

Before we accept a new hypothesis, it must furnish us with distinct credentials, consisting in the deductive anticipation of a series of facts, which are not already connected and accounted for by any equally probable hypothesis. We cannot lay down any precise rule as to the number of accordances which can establish the truth of an hypothesis, because the accordances will vary much in value. While, on the one hand, no finite number of accordances will give entire certainty, the probability of the hypothesis will increase very rapidly with the number of accordances. Seldom, indeed, shall we have a theory free from difficulties and apparent inconsistency with facts. Though one real and undoubted inconsistency would be sufficient to overturn the most plausible theory, yet there is usually some probability that the fact may be misinterpreted, or that some supposed law of nature, on which we are relying, may not be true. Almost every problem in science thus takes the form of a balance of probabilities. It is only when difficulty after difficulty has been successfully explained away, and decisive experimenta crucis have, time after time, resulted in favour of our theory, that we can venture to assert the falsity of all objections.

The sole real test of an hypothesis is its accordance with fact. Descartes' celebrated system of vortices is exploded and rejected, not because it was intrinsically absurd and inconceivable, but because it could not give results in accordance with the actual motions of the heavenly bodies. The difficulties of conception involved in the apparatus of vortices, are mere child's play compared with those of gravitation and the undulatory theory already described. The vortices are on the whole plausible suppositions; for the planets and satellites bear at first sight much resemblance to objects carried round in whirlpools, an

analogy which doubtless suggested the theory. The failure was in the first and third requisites; for, as already remarked, the theory did not allow of any precise cal-. culation of planetary motions, and was so far incapable of rigorous verification. But so far as we can institute a comparison, facts are entirely against the vortices. Newton carefully pointed out that the Cartesian theory was inconsistent with the laws of Kepler, and would represent the planets as moving more rapidly at their aphelia than at their perihelia. Newton did not ridicule the theory as absurd, but showed that it was 'pressed with many difficulties.' The rotatory motions of the sun and planets on their own axes are in striking conflict with the revolutions of the satellites carried round them; and comets, the most flimsy of bodies, calmly pursue their courses in elliptic paths, altogether irrespective of the vortices which they intersect. We may now also point to the interlacing orbits of the minor planets as a new and insuperable difficulty in the way of the Cartesian ideas.

Newton, though he established the best of theories, was also capable of proposing one of the worst; and if we want an instance of a theory decisively contradicted by facts, we have only to turn to his views concerning the origin of natural colours. Having analysed, with incomparable skill, the origin of the colours of thin plates, he suggests that the colours of all bodies and substances are determined in like manner by the size of their ultimate particles. A thin plate of a definite thickness will reflect a definite colour; hence, if broken up into fragments it will form a powder of the same colour. But, if this be a sufficient explanation of coloured substances, then every coloured fluid ought to reflect the complementary colour of that which it transmits. Colourless transparency arises, i 'Principia,' bk. II. Sect. ix. Prop. 53.

k Ibid. bk. III. Prop. 43. General Scholium.

according to Newton, from all the particles being too minute to reflect light; but if so, every transparent substance should appear perfectly black by reflected light, and, vice versa, every black substance should be transparent. Newton himself so acutely felt this last difficulty as to suggest that true blackness is due to some internal refraction of the rays to and fro, and an ultimate stifling of them, which he did not attempt further to explain. Unless some other process came into operation, neither refraction nor reflection, however often repeated, would destroy the energy of light. The theory gives no account, therefore, as Brewster shows, of 24 parts out of 25 of the light which falls upon a black coal, and the 7th part which is reflected from the lustrous surface is equally inconsistent with the theory, because fine coal-dust is almost entirely devoid of reflective power'. It is now generally believed that the colours of natural bodies are due to the unequal absorption of rays of light of different refrangibility.

25

Experimentum Crucis.

As we deduce more and more conclusions from a theory, and find them verified by trial, the probability of the theory increases in a most rapid manner; but we never escape the risk of error altogether. Absolute certainty is beyond the power of inductive investigation, and the most plausible suppositions may ultimately be proved false. Such is the groundwork of similarity in nature, that two very different conditions may often give closely similar results. We sometimes find ourselves therefore in possession of two or more hypotheses which both agree

1 Brewster's 'Life of Newton,' 1st edit. chap. vii.