460 



NATURE 



{Sept, 21, 187 



(Ire?s, tlie Vis Vtva, J'is Moiiuci, and Vis Acceleralrix of that 

 time have, in some of their Protean shapes, hooked themselves 

 like Entozoa into the great majority of our text-books. 



before dealing more definitely with the proper meaning of the 

 woid "Force" I must briefly consider how we become ac- 

 quainted with the physical world, and how consequently it is 

 more than probable that some of our most profound impressions, 

 if uninformed, are completely erroneous and misleading. 



In dealing with physical science it is absolutely necessary to 

 keep well in view the all-important principle that — 



Ntthing can be learned as to the physical world save by obser- 

 vation and expo iment, or by mathematical deductions from data 

 so obtained. 



On such a text, volumes might be written ; but they are un- 

 necessary, for the student of physical science feels at each suc- 

 cessive stage of his progress more and more profound conviction 

 of its truth. He must receive it, at starting, as the unanimous 

 conclusion of all who have in a legitimate manner made true 

 physical science the subject of their study ; and, as he gradually 

 gains knowledge by this — the only — method, he will see more 

 and more clearly the absolute impotence of all so-called meta- 

 physics, or a priori reasoning, to help him to a single step in 

 advance. 



Man has been left entirely to himself as regards the acquire- 

 ment of physical knowledge. But he has been gifted with various 

 senses (without which he could not even know that the physical 

 world exists) and with reason to enable him to control and under- 

 stand their indications. 



Reason, unaided by the senses, is totally helpless in such 

 matters. The indications given by the senses, unless interpreted 

 by reason, are utterly unmeaning. But when reason and the 

 senses work harmoniously together, they open to us an absolutely 

 illimitable prospect of mysteries to be explored. This is the test 

 of true science — there is no resting-place — each real advance 

 discloses so much that is new and easily accessible that the inves- 

 tigator has but scant time to co-ordinate and consolidate his 

 knowledge before he has additional materials poured into his 

 store. 



To sight without reason, the universe appears to be filled 

 with light — except, of course, in places surrounded by opaque 

 bodies. 



Reason, controlling the indications of sense, shows us that the 

 sensation of light is our own property ; and that what we under- 

 stand by brightness, &c., does not exist outside our minds. It 

 shows us also that the sensation of colour is purely subjective, 

 the only difference possible between different so-called rays of 

 light outside the eye being merely in the extent, form, and 

 rapidity of the vibrations of the luminiferous medium. 



To hearing, without reason, the air of a busy town seems to 

 be filled with sounds. Reason, interpreting the indications of 

 sense, tells us that if we could SEE the'particles of air, we should 

 observe among them simply a comparatively slow agitation of 

 the nature of alternate compressions and dilatations superposed 

 upon their rapid motions among one another. And our classifi- 

 cation of sounds as to loudness, pitch, and quality, is merely the 

 subjective correlative of what in the air- particles is objectively 

 the amounts of compression, the rapidity of its alternations, and 

 the greater or less complexity of the alternating motion. 



A blow from a stick or a stone produces pain and a bruise ; 

 but the motion of the stick or stone before it reached the body is 

 as different from the sensation produced by the blow as is the 

 alternate compression and dilatation of the air from the sensation 

 of sound, or the etherial wave-motion from the sensation of 

 light. 



Hence to speak, as the great majority even of " educated " 

 people do, of what we ordinarily mean by light or sound, as 

 existing outside ourselves, is as absurd as to speak of a swiftly- 

 moving stick or stone as pain. But no inconvenience is occa- 

 sioned if we announce the intention to use the terms light and 

 sound for the objective phenomena, and to speak of their sub- 

 jective effects as "luminous impressions" or "noise," as the 

 case may be. In this case there is outside us energy of motion 

 of every kind, but in the mind mere corresponding impressions 

 of brightness and colour, noise or harmony, pain, &c., &c. 



As another instance, it is obvious that we must be extremely 

 cautious in our interpretation of the immediate evidence of our 

 own senses as to heat. 



Touch, in succession, various objects on the table. A paper- 

 weight, especially if it be metallic, is usually cold to the touch ; 

 books, paper, and especially a woollen table-cover, comparatively 

 warm. Test them, however, by means of a thermometer, not 



by the sense of touch, and in all probability you will find litt 

 or no difference in what we call their temperatures. In fact, ar 

 number of bodies of any kind shut up in an inclosure (with 

 which there is no fire or other source of heat) all tend to acqui 

 ultimately the same temperature. Why, then, do some fe 

 cold, others warm to the touch ? 



The reason is simply this — the sense of touch does not infor 

 us directly of temperature, but of the rate at which our fing 

 gains or loses heat. As a rule bodies in a room are colder th: 

 the hand, and heat always tends to pass from a warmer to 

 colder body. Of a number of bodies, all equally colder th: 

 the hand, that one will seem coldest to the touch which is ab 

 most rapidly to convey away heat from the hand. The questio 

 therefore, is one of conduction of heat. And to assure ourselv 

 that it is so, reverse the process : let us, in fact, try , 

 experiment, though an exceedingly simple one ; for the essen 

 of experiment is to modify the circumstances of a physic 

 phenomenon so as to increase its value as a test. Put the pap; 

 weight, the books, and the woollen table-cloth into an oven, a 

 raise them all to one and the same temperature— consideral 

 above that of the hand. The woollen cloth will still be co 

 paratively cool to the touch, while the metal paper-weight m 

 be much too hot to hold. The order of these bodies, as to wa 

 and cold, in the popular sense, is in fact reversed ; and this is 

 because the hand is now receiving heat from all the varic 

 bodies experimented on, and it receives most rapidly from th( 

 bodies which in their previous condition were capable of t 

 stracting heat most rapidly. However it may be in the mo 

 world, in the physical universe the giving and taking powers 

 one and the same body are strictly correlative and equal. j 



Thus the direct indications of sense are in general uttej 

 misleading as to the relative temperatures of diff'erent bodies. 



In a baker's oven, at temperatures far above the boiling poi 

 of water (on one occasion even 320" F., so high indeed thalj 

 beef-steak was cooked in thirteen minutes), Tilletin France, aj 

 Blagden and Chantrey in England, remained for nearly an h(j: 

 in comparative comfort. But though their clothes gave them 5 

 great inconvenience, they could not hold a metallic pencil-c 

 without being severely burned. ' 



On the other hand, great care has to be taken to cover vn 

 hemp, or wool, or other badly conducting substance, every piij 

 of metal which has to be handled in the intense cold to wh 

 an Arctic expedition is subjected ; for contact with very c, 

 metal produces sores almost undistinguishable from burns, thoi< 

 due to a directly opposite cause. Both of these phenonie 

 however, ultimately depend on the comparative facility w 

 which heat is conducted by metals. 



Even from the instance just given, you cannot fail to see t 

 there is a profound distinction between heat and temperatu 

 Heat, whatever it may be, is something which can be tra 

 ferred from one portion of matter to another ; the considerat 

 of temperatures is virtually that of the mere conditions wh 

 determine whether or not there shall be a transfer of heat, .' 

 in which direction the transfer is to take place. Bear this ca, 

 fully in mind, because it has most important analogies to 

 results we meet with in considering the nature of Force. 



It has been definitely established by modern science that h: 

 though not material, has objective existence in as complete a st 

 as matter has. 



This may appear, at first sight, paradoxical ; but we m 

 remember that so-called paradoxes are merely facts as yet un 

 plained, and therefore still apparently inconsistent with oth 

 already understood in their full significance. 



When we say that matter has objective existence, we mi 

 that it is something vi^hich exists altogether independently of 

 senses and brain-processes by which alone we are informed 

 its presence. An exact or adequate conception of it, if it co 

 be formed, would probably be something very different from i 

 conception which our senses will ever enable us to form ; 

 the object of all pure physical science is to endeavour to gr 

 more and more perfectly the nature and laws of the exter 

 world, using the imperfect means which are at our commanc 

 reason acting as interpreter as well as judge, while the senses 

 merely more or less untrustworthy and incompetent witness 

 but still of inconceivable value to us because they are our o 

 available ones. 



Without further discussion we may state once for all that 

 conviction of the objective reality of matter is based mainly uf 

 the fact, discovered solely by experiment, that we cannot in 

 slightest degree alter its quantity. We cannot destroy, nor ( 

 we produce, even the smallest portion of matter. But rea; 



