Swift would have related as the reveries of Laputan philosophy. The Cavendishes and Wollastons of a prior generation, who shrunk back with a certain distrust and alarm even from their own discoveries, are now nowhere to be found. It may be admitted that many of what once appeared insuperable barriers have been removed, and that it is frequently as rash in science to impose limits as to seek to penetrate beyond them. Yet the few single words, Space, Time, Matter, Force, Motion, and Life bring us into direct contact with problems which, though based on innumerable phenomena, forming the totality of our physical knowledge, leave reason utterly at fault. Take for instance the old question regarding that very Matter itself, which we are now so boldly handling, through the properties of its ultimate atoms and molecules. Is it actually created by the same Supreme Power which formed it into worlds and living existences? Or is it in itself eternal-the primitive material with which the Creator has thus wonderfully worked in evoking all that we see in the universe around us? It is obvious that reason is vainly spent in seeking to encounter a question where, though one of the alternatives must necessarily be true, no proof or argument can possibly be brought to determine which is so.

The same with regard to the Infinite, whether of Space, Time, or Number. The mathematician may give technical expression to it, in certain forms to which his science conducts him, and the metaphysician may revel in the very vagueness of the conceptions it conveys; but it is a word unreal to all thought, and philosophy is bound to be sparing in the use of it. It might be well too were Theology, in dealing with these terms of Infinity and Eternity, more thoughtful and forbearing on the doctrines and denunciations to which it applies them. Eternity has been well described as a negative idea clothed with a 'positive name.' Conceptions so vast are, in fact, only described by negative terms-the endless, the incomprehensible. We are all more or less enslaved by words; but it is the proper business, equally of religion and philosophy, to throw off this thraldom, when truth, as often happens, is fettered or distorted by it.

We have just named Matter, Force, Motion, and Life, as terms which in their most general sense give foundation to all science, and at the same time express its most profound and perplexing problems. The word Force especially, known to us through its relation to Matter and Motion in Space, taxes the thought by a sort of harsh compulsion of use. It is a term too variously familiar in common life to be thus largely appropriated by

science. No present definition has rescued it, in this higher sense, from a certain metaphysical obscurity of meaning. We know Force as a reality only by what we term its effects; and we pluralise the word in speaking of the several Forces manifested in the phenomena of the natural world-while at the same time finding, in these very phenomena, a correlation, by interchanges of material effects, so exactly equivalent that nothing which we can term Force or Power is lost in the translation. In this latter fact-one of the greatest discoveries of modern science-we gain a certain unity for the problem, in the conception of a single Power which, indestructible in itself, acts in different modes and degrees throughout the material universe-the source of all motion and change in the greatest and in the most minute phenomena of nature. But this at best is a cloudy conception, insusceptible of any direct proof, and incapable of being moulded into a definition. The abstract idea looms before us, but escapes before we can grasp it.

Nor can we shelter our ignorance under any of the various terms used by philosophers to designate this power-duváμsis, évépyɛta, vis viva, vis mortua, dynamic energy, potential energy, lebendige Kräfte,' or whatever else the diversities or impotence of language have suggested. These phrases, even were they congruous, do little more than repeat the problem in new words. We are still dealing with what is unperceived by any of our senses-itself, for aught we can tell, immaterial-and known only as the cause of sensible changes in the Matter around us. Nor do we gain much here by seeking, as some have done, to conceive of Force as a mere expression of the intestine changes which Matter itself, in its atomical parts, is ever undergoing, and which are in perpetual translation and interchange from one material form to another. This is shifting the difficulty without solving it. Whence come these motions and innumerable interchanges in Matter? What is the power initiating and propagating them? To say that it is one inherent in Matter itself thickens rather than dispels the darkness. M. Laugel enters into these questions, and we give the following passage as a good example of his style :

Elle

'La force est ce qu'il y a de plus mystérieux dans la nature. est dans la substance et n'est pas la substance; ou plutôt la substance étant perpétuellement active et passive, en tant que passive elle subit l'action de la force, en tant qu'active elle devient force à son tour. Car il ne faut point imaginer la force comme quelque chose d'extérieur à la matière ordinaire, comme une entité d'une espèce particulière qui se mêlerait aux corps, y entrerait, en sortirait, au gré des circonstances. Avant qu'on eût bien compris le caractère de l'universalité de la force, telle était l'idée qu'on se faisait des forces particulières. On parlait

du fluide électrique, du calorique, de la gravité, comme d'essences réelles, sur-ajoutées en quelque sorte à la matière. Le langage de la physique n'est pas encore débarrassé de ces locutions vicieuses.'

M. Laugel here and elsewhere shows the intrinsic difficulties of the subject, but provides no new or feasible way out of them. The science of our day has instructed us largely, though yet imperfectly, in the atomic and molecular properties of Matter; and in those multiform changes by addition, subtraction, and substitution on which Chemistry, as a special branch of knowledge, is founded. But it tells nothing of that secret motive cause on which these changes depend, and by which they are translated from one portion of Matter to another, under exact equivalents of power and effect.

It is not surprising that this problem of Force, as grand as obscure, presenting itself in naked form even to the rudest intelligence, should have been seized upon with avidity in all ages. Some of the questions just denoted struck the ancient philosophers as they do us, and were answered with even greater audacity from the absence of those checks which inductive science imposes. The terms 7ò Táσɣov and тò TOLODV briefly τὸ πάσχον ποιοῦν express the relation of Matter and Force in the Greek philosophy. Cicero and Seneca both denote the points in question clearly and compendiously. The science of our own time, though it illustrates these relations in a thousand ways unknown before-though it may be said to have added a new element of power to those already known, and by gigantic efforts of human genius to have converted all to the practical uses of man-yet, as regards the internal nature of Matter and Force severally, has scarcely carried our knowledge beyond that of our predecessors. Motion and change show us the results of their relation, and with these science has its dealings, leaving still open the cardinal question, What is Matter? What is Force? Some philosophers, as we have seen, standing on the brink of these profound problems, merge all Matter in centres and lines of Force; others see Force only in the conditions and changes of Matter itself. We have half-a-dozen books and papers lying before us in which this question is handled, under various conceptions of the points in dispute. And many others are announced as about to appear.

In the recent multiplicity of these writings on Force, as an element in the natural world, we find justification for thus discussing the subject. The ambiguities besetting the term in its various relations have been rather multiplied than lessened by conflicting championship. Even in the case of Heat as a force this comment has its application. This great power, so essen

tial to life and all existence on earth, is now deemed to be a mode of motion of Matter itself; and its variations to depend on interchanges of such atomic motions, tending to equalise their degree, or cause their conversion into mechanical or other kinds of force. The main fountain of Heat to us, as well as of Light, is the Sun. This great body projects, through the ether of intervening space, waves or impulses, so variously and wonderfully propertied as to produce, on reaching the earth, those several effects of light, heat, and chemical action, of which the solar spectrum is the simple but sublime interpreter. To the Sun, then, we must look for that astonishing initial force, whatever it be, which from age to age combines and emits those complex undulations of which Heat and Light are the exponents to us on earth, while they alike pervade every part of the solar system. We may admit that Heat, as expressed by temperature in the grosser forms of matter, is simply due to intestine movements of their particles; but we cannot exclude the Sun as the present primary source of that power which these motions distribute and equalise. The discoveries of Tyndall show by what subtle molecular adjustments the heat thus received is prevented from freely radiating back into space. The question whether the sun loses by this unceasing emission of power-for we are not authorised to call it substance -and how this loss, if real, is repaired, have been subjected to various recent hypotheses, but without any certain or even plausible conclusion. If indeed the notion of necessary repairbe admitted, we are called upon to provide for more than two million times the amount which the sun transmits to the earth, such being the relative proportion of this power lost-if lostby projection into circumambient space.

Latent Heat again-or what we are called upon to regard as synonymous, latent force or potential energy-is among the conceptions which modern science has embodied in its doctrines; a difficult conception, indeed, but based on the apparent phenomena of bodies passing successively through the solid, fluid, and gaseous states. Even if Dr. Andrews' recent discoveries did not throw doubt on the interpretation of these phenomena, we should still have to ask, What is this latent force of Heat? The name implies an existing reality. In what does this reality consist? Theory can only answer, In some interior specific condition or arrangement of atoms, lasting until excited to fresh change. But see how much obscurity hangs over all this, when closely analysed! How much obscurity, too, in that general conception of potential force or energy stored up in matter, which furnishes so many startling illustrations to

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the scientific teaching of the day. It is deemed possible to say that Heat and Light, as forces or active powers, absorbed originally from the sun by vegetable life on the earth, and following the conversion of the latter into coal, have thus lain dormant for untold ages in a mineral form, to be finally extricated in the fires and furnaces of our own time. We cannot disprove this, or bring other hypothesis to meet the facts. But when we speak of heat as a force, consisting integrally in certain atomic motions of bodies, which force may be pent up for ages in these atomic recesses, yet ever ready for extrication, we are bound to look fairly at the abstract conceptions these things involve, if indeed they can be truly understood in any other way than as simply expressing phenomena. The word Force, with all the adjuncts imposed upon it, still looms before us, as a mysterious symbol rather than an intelligible reality.

We have been led to dwell long on this subject from feeling that the conception of Force-the very backbone, we may call it, of physical science-has been grievously disjointed by the various and vague use made of the term. Whether any word

or phrase could be devised giving more unity to the idea, and to the phenomena it embodies, may be doubtful. We do not ourselves venture to suggest one. The radical difficulty lies in the mysteries of nature itself, which we have not sufficiently penetrated to draw this unity from their depths. Such difficulty becomes more manifest as we pursue the subject into other of its ramifications. If we do so here, it is less for the purpose of exposing the deficiencies of our knowledge than to show what science has done, or is yet seeking to do, in the several cases where Force is brought in as the exponent of phenomena.

We pass over mechanical forces, though to these also some of the foregoing remarks will apply. Coming to Gravitation, we are on smoother ground as regards the sequence of facts and the phraseology expressing them, though still ignorant of the intimate nature of this great power of the universe. Unlike other forces in the sublime simplicity of its laws, this very simplicity becomes a bar to research. The legacy of ignorance which Newton left behind him, declaring, with the wonted candour of genius, that he did so, has descended to his successors in the inquiry, who must, in their turn, bequeath it to posterity. Several mathematicians and experimentalists of our own time-Faraday among the latter-have adventured on the research, with the especial object of bringing Gravity into some direct relation with the other forms of force, but hitherto in