334 



NATURE 



\_Atigtist 4, 1887 



■necessary to make Kepler's laws fit observation exactly. We 

 are inevitably led to regard the attraction of gravitation as the 

 ■cause which keeps the planets in their orbits. 



But it may be said, what is the difference in the two cases ? 

 Is not the law of gravitation merely a simpler mode of express- 

 ing the observed facts of the planetary motions just like the 

 somewhat less simple laws of Kepler ? What right have we to 

 introduce the idea of causation in the one case more than in 

 the other ? 



The answer to this appears to be that in the one case, that of 

 Kepler's laws, supposing them to be true, we have merely a 

 statement of what, on that supposition, would be a fact regard- 

 ing the motions of the planets, whereas in the other case the 

 observed motions are referred to a property of matter of the 

 operation of which in other and perfectly different phenomena 

 we have independent evidence. 



I have purposely omitted to mention the important difference 

 between the two cases, which lies in the circumstance that 

 Kepler's laws require correction to make them applicable to long 

 intervals of time, whereas the law of gravitation shows no sign 

 of failure ; because, even if the former had been perfectly exact, 

 however long the interval of time to which they were applied, I 

 doubt if they would have carried with them the idea of causation. 

 To take another simple illustration, let us think of the pro- 

 pulsion of a bullet in an air-gun. We speak of the motion of 

 the bullet as being caused by the elasticity of the compressed 

 air. And the idea of causation comes in because we refer this 

 particular instance of motion to a property of gas, of the 

 existence and operation of which we have evidence in perfectly 

 independent phenomena. 



It is thus that in scientific investigation we endeavour to 

 ascend from observed phenomena to their proximate causes ; 

 but, when we have arrived at these, the question presents it- 

 self, can we in a similar manner regard these causes in turn as 

 themselves the consequences of some cause stretching still 

 further back in the chain of causation ? If the motion of the 

 bullet in an air-gun be caused by the elasticity of the com- 

 pressed air, can we account for the elasticity of a gas? If 

 the retention of the planets in their orbits be due to the 

 attraction of gravitation, can we explain how it is that two 

 material bodies should attract one another across the intervening 

 space ? 



Till a time well on in the present century, we could only take 

 the elasticity of gases as a fact, and deduce the consequences 

 which flow from it. But the researches of Joule and Clausius 

 and Maxwell and Crookes and others have accumulated so 

 much evidence in favour of the general truth of the kinetic 

 theory of gases, that we are now disposed not to rest in the 

 elasticity of gases as an ultimate property beyond which we 

 cannot go, but to regard it as itself a consequence of the mole- 

 cular constitution of bodies, and of the motions and mutual 

 collisions of the ultimate molecules of a gas. Respecting the 

 attraction of gravitation we have not at present made a similar 

 advance. Speculations, indeed, have not been wanting on the 

 part of those who have endeavoured to account for it. But 

 none of these so fits into the known phenomena of Nature as to 

 carry with it a conviction of its truth. Yet there is one in- 

 dication that though we cannot at present explain the cause of 

 gravitation, yet it may be explicable by what are called second 

 causes. The mass of a body is measured by its inertia ; and, 

 though we commonly think of a body of large mass as being 

 heavy, and though we compare the masses of two bodies most 

 easily and accurately through the intervention of weight, yet 

 the idea of mass may be acquired, and means might easily be 

 suggested by which the ratio of the masses of two bodies might 

 be experimentally determined, without having recourse to gravi- 

 tation at all. Now, according to the law of gravitation, the 

 force with which a given body attracts another at a given dis- 

 tance is strictly proportional to the mass of the latter. If we 

 suppose the attracting body to be the earth, and the attracted 

 bodies to be in one case a brass weight, and in the other a piece 

 of marble, it follows that if they make equilibrium when placed 

 in the pans of a true balance — I make ab traction of the effect 

 of the buoyancy of the air — their masses are strictly equal, and, 

 accordingly, that weight is a true measure of mass. But 

 there is no reason a priori, so far as with our present knowledge 

 we can see, w hy this should be so. We know that if the bodies 

 in the scale-pans were formed, one of brass and the other of 

 iron, and there were a magnet concealed under the table on 

 which the operator placed his balance, the masses would not be 



equal when there was equilibrium. But that the law is true, 

 and that, accordingly, weight is a true measure of mass, follows 

 with the highest probability from the third of Kejler's laws, and 

 was proved experimentally by Newton, by experiments with 

 pendulums. Newton's experiment has since been repeated by 

 Bessel, with all the refinements of modern appliances, with the 

 result that so far as the most exact experiments enable us to 

 decide the law is strictly true. This is perhaps the only instance, 

 as Sir William Thomson remarked to me in conversation, in 

 which there is an exact agreement between two quantities, and 

 yet we are unable to give any reason why they should agree. That 

 such is the case, holds out some prospect of scientific men being 

 able some day to explain gravitation itself — that is, to explain it 

 as the result of some still higher law. 



Such is the nature of our progress in scientific investigation. 

 We collect facts ; we endeavour to co-ordinate them and ascer- 

 tain the laws which bind them together ; we endeavour to refer 

 these laws to their proximate causes, and to proceed step by 

 step upwards in the chain of causation. Presently we arrive at 

 a i-tage at which, even after long trial, we do not see our way to 

 going further. Yet we are not able to demonstrate that further 

 progress in the same direction — that is, along the cham of 

 secondary causation — is impossible. Science conducts us to a 

 void which she cannot fill. 



It is (in other grounds that we are led to believe in a Being 

 who is the Author of nature. A conclusion so important to 

 mankind in general is not left to be established as the result of 

 investigations which few have the leisure and ability to carry 

 out. Doubtless, where it is accepted, the study of science en- 

 larges our ideas respecting the greatness of that Being, and 

 tends to keep in check notions of too anthropomorphic a 

 character which we might form concerning Him. Still, the 

 subject-matter of scientific study is not, at least directly, theistic, 

 and there have not been wanting a few instances of eminent 

 scientists who not merely rejected Christianity, but apparently 

 did not even believe in the being of a God. 



The religious man, on the other hand, who knows little or 

 nothing of science, is in the habit of contemplating the order 

 of Nature not merely as the work of God, but in very great 

 measure as his direct work. Of course, the concerns of every- 

 day life present innumerable instances of the sequence of cause 

 and effect ; and few are now so ignorant of the very elements 

 of science as not to allow that the sequence of day and night, 

 of summer and winter, is proximately due to the rotation of the 

 earth about its axis, and the oblique position of that axis with 

 re''erence to the plane of the earth's orbit. But when we get 

 beyond the region of what is familiarly known, still more when 

 we get outside the limits of well-ascertained scientific conclusions, 

 and enter a region which is still debatable ground, where men of 

 science are attempting to push forwards, and are framing 

 hypotheses with a view to the ultimate establishment of a theory 

 in case those hypotheses should stand the test of thorough 

 examination, — when, I say, we get into this region, a man such as 

 I have supposed may feel as if the scientists who were attempting 

 to explore it were treading on holy ground ; he may mentally 

 charge them with irreverence ; perhaps he may openly speak of 

 them in a manner which implies that he attributes to them an 

 intention to oppose revealed religion. 



To take a particular example. I can imagine that a man such 

 as I have supposed may have always been in the habit of regard- 

 ing each one of the thousands and tens of thousands of species 

 into which naturalists have divided the animal and vegetable 

 kingdoms as having originated in an independent creative act ; 

 that the supposition may have become entwined among his 

 religious beliefs. Such a man would be apprehensive of any 

 attempt to introduce second causes in explanation of the observed 

 fact of the great multiplicity of species. 



Akin to the feeling which I have attempted to describe is 

 another, against which we must be on our gu ird. The religious 

 man is strongly impressed with the truth of certain things which 

 lie outside the discoveines of reason or the investigations of 

 science, and which bear on the whole conduct of his life here, 

 and on his hopes regarding a life hereafter. He believes these 

 truths to be divine, and, accordingly, that no legitimate deduc- 

 tion of human reason is liable to come in conflict with them. 

 But the precise mode in which a conviction of the truth of these 

 things was arrived at depends, to a considerable extent, on each 

 man's idiosyncrasy. His natural bent of mind, his early training, 

 his later associations, have all a good deal to do with it. Divine 

 truth is one thing ; our own apprehension of it, and the steps by 



