Sept. 21, 1876] 



NATURE 



463 



fore be called radiant heat or ligLt) or produce chemical decom- 

 position, as of carbonic acid and water in the leaves of plants, 

 or of silver salts in photography (and be therefore called acti- 

 nism) ; the energy of motion of the particles of a gas, upon 

 which its pressure depends, &c. [When the motion is vibratory 

 the energy is generally half potential, half kinetic] 



These explanations and definitions being premised, we can 

 now translate Newton's words (without alteration of their mean- 

 ing) into the language of modem science, as follows : — 



Work done on any system of bodies (in Newton's statement the 

 parts of any machine) has its equivalent in work done against 

 friction, molecular forces, or grazity, if there be no acceleration ; 

 but if there be acceleration, part of the work is expended in over- 

 coming the resistance to acceleration, and the additional kinetic 

 energy developed is equivalent to the work so spent. 



But we have just seen that when work is spent against mole- 

 cular forces, as in drawing a bow or winding up a spring, it is 

 stored up as potential energy. Also it is stored up in a similar 

 form when done against gravity, as in raising a weight. 



Hence it appears that, according to Newton, whenever work 

 is spent it is stored up either as potential or as kinetic energy, 

 except, possibly, in the case of work done against friction, about 

 whose fate he gives us no information. Thus Newton expressly 

 tells us that (except, possibly, when there is friction) work is 

 indestructible, it is changed from one form of energy to another, 

 and so on, but never altered in quantity. To make this beautiful 

 statement complete, all that is requisite is to know what becomes 

 of work spent against friction. 



Here, of course, experiment is requisite. Newton, unfor- 

 tunately, seems to have forgotten that savage men had long since 

 been in the habit of making it whenever they wished to procure 

 fire. The patient rubbing of two dry sticks together, or (still 

 better) the drilling of a soft piece of wood with the slightly 

 blunted point of a hard piece, is known to all tribes of savages 

 as a means of setting both pieces of wood on fire. Here, then, 

 heat is undoubtedly produced, but it is produced by the expen- 

 diture 0/ work. In fact work done against friction has its equi- 

 valent in the heat produced. This Newton failed to see, and 

 thus his grand generalisation was left, though on one point only, 

 incomplete. The converse transformation, that of heat into 

 work, dates back to the time of Hero at least. But the know- 

 ledge that a certain process will produce a certain result does 

 not necessarily imply even a notion of the "why;" and Hero 

 as little imagined that in his seolipile heat was converted into 

 work, as do savages that work can be convtrted into heat. 



But whenever any such conversion or transference takes place 

 there is necessarily motion : and the mere rate of conversion or 

 transference of energy per unit length of that motion is, in the 

 present state of science, very conveniently called force. No 

 confusion can arise from using such a word in such a sense. 

 On the contrary, there is always a gain in clearness when com- 

 pactness can lawfully be introduced, 



Rumford and Davy, at the very end of last century, by totally 

 different experimental processes, showed conclusively that the 

 materiality of heat could not be maintained, and thus gave the 

 means of completing Newton's statement which, still farther 

 extended and generalised rather more than thirty years ago by 

 the magnificent experimental work of Colding and Joule, now 

 stands as one massive pillar of the fast-rising temple of science: — 

 known as the law of the conservation of energy. 



The conception of kinetic energy is a very simple one, at 

 least when visible motion alone is involved. And from motion 

 of visible masses to those motions of the particles of bodies 

 whose energy we call heat, is by no means a very difficult mental 

 transition. Mark, however, that heat is not the mere motions 

 but the energy of these motions ; a very different thing, for heat 

 and kinetic energy in general are no more " modes of motion " 

 than potential energy of every kind (including that of unfired 

 gunpowder) is a " mode of rest!" In fact a *' mode of motion " 

 is, if the word motion be used in its ordinary sense, purely kine- 

 matical, not physical ; and if motion be used in Newton's sense, 

 it refers to momentum, not to energy. 



The conception of potential energy, however, is not by any 

 means so easy or direct. In fact, the apparently direct testi- 

 mony of our muscular sense to the existence of force makes it at 

 first much easier for us to conceive of force than of potential 

 energy. Why two masses of matter possess potential energy 

 when separated — in virtue of which they are conveniently said 

 to attract one another — is still one of the most obscure problems 

 in physics, I have not now time to enter on a discussion of the 

 very ingenious idea of the ultramundane corpuscles, the out- 



come of the Itfe-work of Le Sage, and the only even apparently 

 hopeful attempt which has yet been made to explain the 

 mechanism of gravitation. The most singular thing about it is 

 that, if it be true, it will probably lead us to regard all kinds of 

 energy as ultimately kinetic. 



And a singular quasi-metaphysical argument may be raised 

 on this point, of which I can give only the barest outline. The 

 mutual convertibility of kinetic and potential energy shows that 

 relations of equality (though not necessarily of identity) can exist 

 between the two, and thus that their proper expressions involve 

 the same fundamental units, and in the same way. Thus, as 

 we have already seen that kinetic energy involves the unit of 

 mass and the square of the linear unit directly, together with 

 the square of the time unit inversely, the same must be the case 

 with potential energy ; and it seems very singular that potential 

 energy should thus essentially involve the unit of time if it do 

 not ultimately depend in some way on energy of motion, 



[Prof. Tait then gives instances of the inaccurate use of the 

 word Force.] 



To conclude — In defence of accuracy, which is the sine qudnon 

 of all science, we must be "zealous," as it were, even to " slaying." 

 And, as all the power of the Times will not compel us to put a 

 y instead of an e into the word chemist, so neither will the bad 

 example of Germany and France, though recommended to us 

 with all the authority which may be attributed to an ex-president 

 of this Association, succeed in inducing us to attach two or more 

 perfectly distinct and incompatible scientific meanings to that 

 useful little word, "force," which Newton has once and for ever 

 defined for us with his transcendent clearness of conception. 



I have now only to ask your indulgence for the crudeness 

 of this lecture. All I can say is that in preparing it, I have 

 done my best, under circumstances of time, place, and surround- 

 ings, all alike unpropitious. But the chance of being able to 

 back up, however imperfectly, my old friend, Dr. Andrews, in 

 whose laboratory I first learned properly to use scientific appa- 

 ratus, and whose sage counsel impressed upon me the paramount 

 importance of scientific accuracy, and above all, of scientific 

 honesty — such a chance was one which no surroundings (how- 

 ever unpropitious) could have induced me to forego. 



NOTES 

 We have received the " Daily Programme of the Twenty- 

 fifth Meeting of the American Association," held at Buffalo, 

 August 23-30. It forms a pamphlet of about 100 pages, but 

 appears to have been published daily during the meetings, and 

 is quite a model of what such a programme should be. It is 

 clearly printed on excellent paper, and has not the overcrowded 

 appearance that the programme of the British Association often 

 presents. At the last meeting a standing committee was ap- 

 pointed to superintend the selection of papers, and to this 

 committee a short abstract must be sent before the title of a 

 paper can be transmitted to the sectional committees. A list 01 

 accepted papers is given each day, and appended is the time 

 each is supposed to occupy in reading. The work of each 

 section for each day is indicated, and all the necessary infor- 

 mation as to officers, r^nlations, &c., are given. A list is also 

 given daily of the number of members " elected " and the number 

 "registered," with their addresses. Altogether for this meeting 

 these amount to 352, and the number of papers entered 

 for reading is 147. At this meeting seventeen fellows were 

 elected, consisting of some of the best known names in American 

 science. The next meeting of the Association will be held at 

 Nashville, Tenn,, on the last Wednesday of August, 1877, the 

 president-elect being Prof, Simon Newcomb, of Washington, 



Pbof, Huxley was present at the meeting of the American 

 Association for the Advancement of Science, held at Buffalo. 

 After stating that he was quite unprepared to occupy their atten- 

 tion, he said : — In England we have no adequate idea of the 

 extent of your country, its enormous resources, the distances 

 from centre to centre of population, and we least of all under- 

 stand the great basis of character which sprung from the other 

 side of the Atlantic. There has been some talk of the influence 

 of your climate carrying you back to ths North American type. 



