54 



SCIENCE 



[N. S. Vol. XLVII. No. 1203 



sions for the forces between elements gave 

 the same results when integrated around 

 closed circuits, and no one succeeded in de- 

 vising experiments which would discrimi- 

 nate between them. Even Ampere, how- 

 ever (like the great predecessor whose 

 name Maxwell connected with his), was not 

 immune from the inherent desire of the 

 physicist for "explanations" of distance- 

 forces, though he was compelled to forego 

 them because no way appeared for putting 

 them to an experimental test. At the be- 

 ginning of the memoir,^ in which he sums 

 up his eleetrodynamic researches, after de- 

 claring his adherence to the Newtonian pro- 

 cedure and renouncing anything in the na- 

 ture of Cartesian vortices which Oersted's 

 discovery had in a measure revived, he 

 says: 



1 have made no attempt to find the cause of these 

 forces, well persuaded that any attempt of this 

 kind ought to be preceded by the purely experi- 

 mental knowledge of the laws and by the determi- 

 nation from these laws alone of the value of the 

 elementary forces, whose direction is necessarily 

 that of the straight line drawn through the ma- 

 terial points between which the forces act. 



Later in the same memoir^ he disclaims 

 any intention to assert that his forces are 

 to be regarded as "truly elementary" and 

 calls attention to previous attempts of his 

 own* to "assign a cause for these forces in 

 the reactions of the fluid filling all space 

 whose vibrations produce the phenomena of 

 light." 



Simultaneously with these developments 

 and partly in consequence of some of 

 them, the employment of imponderable 

 fluids became very general in theoretical 

 physics. In electrostatics and magnetism, 

 the gravitational analogy required some 

 sort of attracting or repelling substance; 



2 Mem. de I'Acad., VI., p. 177 (1825). 



3 P. 294. 



*"Eecueil d 'observations eleetro-dynamiques, " 

 p. 215. 



in the theory of heat, the calprimetrical ex- 

 periments of Black and his clear discrimi- 

 nation between temperature and quantity 

 of heat, led directly to a substantial theory 

 of heat. There was no great encourage- 

 ment for the attempt to apply the prin- 

 ciples of mechanics to these imponderables ; 

 so far as experiment showed they lacked 

 not only the conspicuous property of 

 weight, but also the most essential dynam- 

 ical characteristic of ordinary matter, viz., 

 inertia. The natural and fertile method of 

 dealing with them was to take some em- 

 pirical relations, as simple and funda- 

 mental as possible, as postulates for the 

 mathematical development of the subject. 

 Some of the most epoch-making advances 

 in theoretical physics are instances of this 

 method; as examples one needs only to re- 

 call Fourier's theory of heat conduction 

 (afterwards applied by Ohm to the con- 

 duction of electricity), and Carnot's de- 

 duction of the theory of heat engines from 

 the empirical principle which we now call 

 the second law of thermodynamics. In fact, 

 the great physicists who flourished during 

 the first three or four decades of the nine- 

 teenth century seem to have felt that there 

 was little hope of giving dynamical expla- 

 nations of all physical phenomena. Thus 

 Fourier, in the introduction of his great 

 work, recounting the glorious achievements 

 of Newton and his successors, says : 



It is recognized that the same principles regu- 

 late all the movements of the stars, their form, 

 the inequalities of their courses, the equilibrium 

 and oscillations of the seas, the harmonic vibra- 

 tions of air and sonorous bodies, the transmission 

 of light, capillary action, the undulations of 

 fluids, in fine the most complex effects of all the 

 natural forces; and thus has the thought of New- 

 ton been confirmed; quod tarn paiLcis tarn multa 

 prwstet geometria gloriatur. 



"But," continues Fourier, "whatever 

 may be the range of mechanical theories, 

 they do not apply to the effects of heat. 



