Feb. 28, 1878] 



NATURE 



341 



Maxwell, reviewed by Prof. Tait in Nature (vol. xvi. p. 119), 

 a very moderate acquaintance with which has sufficed to remove 

 from my mind all the doubts and perplexities which, without 

 such assistance, must beset every cultivator of physics and 

 mechanics attempting to take a comprehensive view of these two 

 parallel sciences in their close relations to each other. The 

 latter science especially, mutilated and deformed, and roughly 

 scattered up and down in fragments, as we commonly find it 

 represented, wears in general in our crude brains and in ordinary 

 practice very much the same dismembered aspect which physics 

 in its numerous subordinate branches presents to those who 

 devote their attention especially only to some particular one of 

 its departments. 



But the new and comprehensive science of energy has, besides, 

 its own special debatable region, in much the same way that 

 mechanics has, although of an entirely different description ; 

 and however cheerfully we might consent, by basing all the pro- 

 positions of mechanics (a perfectly possible proceeding, as has 

 here been indicated) upon a system of permanent and reciprocal 

 force-pairs, to include among the vicissitudes of force-action, 

 besides its own clearly distinguishable phenomena, also (with 

 countless impenetrably hidden fields of operation) all the known 

 agencies of its more versatile and less easily definable kindred 

 science of energetics, yet it can scarcely be regarded as imme- 

 diately desirable, in the absence of sufficiently abundant proof, 

 to make this assumption ; nor is it perhaps expedient, on the 

 new account just mentioned, to take it too readily for granted as 

 a sound and simple basis of the leading laws of the new science, 

 until the field of phenomena which the latter are framed to in- 

 clude is itself so clearly defined and circumscribed, as not to offer 

 in its own relations and conditions objections to the course which 

 may seem to contain in them anything which may prove to be 

 insuperable, or which might very quickly lead to its abandonment. 

 To assert the principle of virtual velocities concerning the 

 agent force, although we can voluntarily enlist the action of this 

 agent in mechanical combinations, does not necessarily compro- 

 mise our free will in any way, because the manner of enlisting 

 this servant of our will cannot be definitely, and in a scientific 

 point of view completely specified as the necessary form which 

 the exercise of volition must take ; and accordingly no natural 

 law which completely binds and describes any force, can possibly 

 describe and define also, as completely, the volition which pro- 

 duces it. But even if the volition concerned in producing a 

 force were, as a cause, completely definable, and if we may 

 assume that pure inductive science is capable immediately of so 

 describing it in part, and of ultimately (in its indefinitely achieved 

 development) reaching no partial or imperfect view of every 

 process of volition, so as to be able with assigned actions of will 

 to construct a perfectly unerring plan of all the operations of a 

 Providence subjected to these conditions, and to trace without a 

 single fault or discontinuity the whole current of consequent 

 events belonging to them, yet it is evident that the result would 

 lack an element of genuineness, of whose absence we should 

 immediately be conscious as rendering it an inadequate and 

 unauthentic represesentation of the operations of that perfect 

 will and of that Divine Omnipotence, to whose purposes we 

 owe the obedience and the entire subserviency of our wills in all 

 our actions. This moral obligation of our actions springs from a 

 side of our natures truly unseen, but to which we owe dictates 

 of our actions as quick and spontaneous as those which come 

 endorsed with reason to us from our natural senses. On the 

 other hand, to suppose that reason will ever bridge the gap 

 which divides inanimate from living agency, and will be able to 

 register perfectly on her tablets (in the way just now supposed) 

 every event of volition, is as visionary as to suppose her capable of 

 apprehending and of taking a measurable account of the purposes 

 of those actions which we hold to be inspired. But in the part 

 which reason plays as a faculty given to us for learning wisdom 

 and for seeking after and cultivating virtue from our cradles, in 

 all the vicissitudes of life, there appears to be no break or in- 

 terruption- to its ownward progress, though its goals may be 

 partly invisible and partly unattainable ; and "new forces" in 

 nature must evidently lie abundantly along its path. The 

 "forces" of living beings, in particular, are inscrutable to it, 

 and those of humanity at least must especially be so, for two 

 reasons, a moral, as well as a vital or organic one, both differ, 

 ently descriptive of the ultimate constitution of our free will. 

 If, therefore, there appears no ground (as I believe that Ilirn's, 

 and perhaps other experiments, have shown) for introducing an 

 exception of living agents in the law of conservation of energy, 

 perhaps the progress of physiology and of biological physics 



may also show that to make the same exception in the law of dis- 

 sipation, or of the loss of availability of energy in every action, is 

 equally incapable of substantiation could we see those forms of 

 energy which we, and other living beings, make use of in appa- 

 rently transgressing the generality of this law by partially restor- 

 ing their availability to some very obvious forms of energy. 



In this view of infinite progress of investigation, energy must 

 keep its form of energy of motion, or of such energy converted 

 into work of "agents;" and from what has been above de- 

 scribed, it is not necessary that the work of these agents should 

 be the energy thus abandoned in a neiv kinetic form. All the 

 actions of an agent can be imagined to be consequences of special 

 kinds of motion, but of what advantage it may be to suppose it, 

 when in the midst of conceptions so distractingly profound and 

 unapproachable as encircle the new science of energy, an agent 

 as simple and intell'gible as mechanical force is presented to our 

 understanding as an example of what an agent of will and pur- 

 pose may perhaps be like, it is very difficult to reflect upon and 

 comprehend. 



At the outset of this long-since-begun, and now quiti* 

 differently-concluded letter from what I contemplated,! I pro- 

 posed, in connection with Mr. Crookes' famous series of investi- 

 gations (especially those last crowning points of his discoveries 

 in which vacua so perfect were produced as fairly to eliminate 

 the principal cause of rotation of the arms of a radiometer, 

 originally recognised in the action of residual gas), to point out 

 some means by which, in vacua so complete, the mode of action 

 of force might possibly be elucidated by experiments. A beam 

 of rays, bent and reflected, for example, so as to fall at grazing 

 incidence from the right or left on a flat end, instead of on a vane 

 of one of the arms of a very perfectly- exhausted radiometer, 

 might be found to move it sensibly, and perhaps more distinctly, 

 as the exhaustion reached its limit, in opposite directions cor- 

 responding to the directions from which the beam grazed the 

 face, which it would be difficult to attribute to molecular im- 

 pacts of the residual gas ; and in the action of such an external, 

 and to all ordinary perceptions quite uncounterpoised, force 

 (supposing radiation really to produce it), a field of new dis- 

 coveries relating to direct mechanical effects of the luminiferous 

 ether would obviously present itself, which would be of the 

 highest interest and consequence. But as regards the interpre- 

 tation of any effects which might be observed, especially in con- 

 nection with new views of the nature of potential energy which 

 they might open out, I prefer now to refrain from offering any 

 hints or suggestions, knowing that any inquiry which offers 

 prospects of studying force under a new aspect, cannot be guided 

 and directed beforehand, so as either to establish or confute any 

 of the already well-proved laws of its action, but that in the 

 broad principles which the science of energy presents for our 

 consideration and development it could only be prosecuted as a 

 new science, a new branch of general physics contributing some- 

 thing like its predecessors (heat, radiation, chemical action, 

 electricity, &c. , but tuhat we should attempt in vain to picture to 

 ourselves) in the capacious science of energy, as a new ascent 

 towards that lofty pinancle to which in common with several other 

 natural sciences energetics also proposes to raise itself in the end, 

 to contemplate the Divine works of True Beneficence and to 

 discern in the stately Temples of the Universe the allotted place 

 of man. 



These are some of the teachings of the radiometer which rose 

 up before me when in an unguarded moment I asked myself the 

 question : What change from the point of view of energy con- 

 servation would it introduce into our view of the experiment if, 

 supposing that a force v/ere found to actuate the vane of a radio- 

 meter, which was a direct effect of radiation, we were to sacrifice 

 the integrity of Newton's third law of motion by assuming the 

 existence of a new class of forces which act alone unaccompanied 

 by any equal and opposite reaction ? ^ The answer here must be 

 that if energy is still to be conserved (that is to say, if we can point 

 out the source and destination of all the work that is performed), 

 there must be a law in these outer forces connecting them zvitk 

 knoxvn physical agents in such a manner that as much work is 

 done upon them in any assigned change of configuration as is 

 supplied by those physical agents in the change, and as the 

 internal forces and other agents in the changing system also 

 furnish by their action. (See Prof. Clerk Maxwell's definition of 

 a "conservative system" in " Matter and Motion," p. 59, where 

 the action of internal forces is excluded by supposing the system to 



' Reaction is not meant here, of course, to imply Newton's imaginary 

 "resistance of an acceleration ;" but the real active tendency of some equal 

 opposite force only, is meant to be understood. 



