124 



NATURE 



ytmc 13, 1872 



a curious concatenation of errors. Dr. Joule never em- 

 ployed a weight of 772 lbs., for the weights he employed 

 were all under 30 lbs. ; he never employed tlie energy ac- 

 quired by his descending weights in stirring an e.\act 

 pound of water, for the stirring vessel was not constructed 

 to hold that particular quantity ; neither did he find that 

 the descent of the weight with a given \elocity raised the 

 water exactly I' P~., for ihe velocity of descent has 

 nothing whatever to do with the result ; since precisely 

 the same amount of energy is acquired by a body in de- 

 scending through the space of one foot by the action of 

 gravitation, whether ore minute or one month were occu- 

 pied in the descent. For the sake of convenience, and in 

 order to avoid a source of error, it was desirable only that 

 the weight* should descend slowly, and with a tolerably 

 uniform velocity. 



.So much for the reviewer's knowledge of facts ; now as 

 to his knowledge of theories. He writes (p. 31) :— 



" The supporters of the doctrine of the indestructibility 

 (? conservation) of energy have adopted a method the re- 

 verse of scientific. They start with the assumption of 

 perpetual motion by means of transformation. In order 

 to make facts fit their hypothesis, they take for granted 

 that heat, light, electricity, and magnetism, are modes of 

 motion, but not requiring a material basis to account for 

 their phenomena. Some, however, seem to be aware that 

 motion of necessity implies something moving, and that 

 this something must be matter in some of its forms, and 

 that consequently it is a great mistake to suppose that the 

 dyn.imical theory is inconsistent with the materiality of 

 heat. Finding that they have been a little too hasty in 

 getting rid of the old imponderables, they are now quietly 

 bringing them back under a new name, hoping, doubtless, 

 that their few remaining friends may not be able to re- 

 cognise them. Instead of the 'imponderables' we now 

 ha\e the ' luminiferous ether' which fills stellar space, 

 and even permeates all the grosser forms of material 

 existence." 



This sentence expresses a gross misrepresentation of 

 the course of philosophic thought. Can the reviewer point 

 out a single physicist who for a moment doubts that "mo- 

 tion of necessity implies something moving, and that this 

 something must be matter in seme of its forms ; " it is an 

 axiom that not "some" but all must obviously admit. 

 When it was supposed that light and heat consisted of 

 material particles projected with immense velocity from 

 their radiant sources, and that electricity and magnetisnr 

 were " fluids " travelling with similar velocity, it might be 

 assumed that inter-stellar space is an absolute vacuum ; 

 but when the progress of physical knowledge developed 

 phenornena which were partially or wholly incapable 

 of explanation on this hypothesis (such as diffraction and 

 interference, and subsequently those of the polarisation of 

 light and heat) ; but which became perfectly intelligble 

 on the hypothesis that these forms of energy consisted 

 in vibratory molecular motion transmiitted with the same 

 great velocity, the existence of a highly attentuated and 

 elastic medium as the denizen of infinite space, became a 

 necessary part of the theory ; and this, in unavoid- 

 able ignorance of its precise nature, was termed "ether." 

 Hence, in direct opposition to the reviewer's statement, 

 physicists take for granted that " light, heat, electricity, 

 and magnetism " do require a " material basis " (that is, 

 matter as a means of their transmission) "to account for 

 their phenomena;" and his dictum about physicists 

 getting rid of the old imponderables, and now bringing 

 them back again, is unmitigated nonsense. 



The attribute of imponderability has been ascribed to 

 the hypothetical substance "ether" by many physicists who 

 hold that it monopolises the property of transmitting the 

 waves of light and heat, and is therefore intersticially de- 

 posited in all kinds of matter. The writer is, however, 

 more inclined to believe, with Mr. Justice Grove, that all 

 kinds of matter are susceptible of these vibratory motions. 



and hence that the hypothesis of iiilt-rsticial ether is 

 gratuitous ; his reasons for such opinion being elsewhere 

 m print, need not be here repeated.* 



It may be further suggested that for all that is positively 

 known to the contrary, all kinds of matter may possibly 

 be susceptible of a fourth state or condition, which may 

 be termed the "ethereal," and which in tenuity and elas- 

 ticity may be as far beyond the gaseous, as the gaseous is 

 beyond the fluid state ; possessing also the mechanical 

 properties of a jelly, rather than those of a gas. 



The writer can hardly be expected to take up the 

 cudgels for others against the reviewer ; but in concluding 

 the remarks on this point, it may be observed regarding 

 the reviewer's eniph.itic denunciation of an alleged des- 

 crepancy between Grove and Tyndall, that the discrepancy 

 exists only in his own misunderstanding the quotation 

 from the " Correlation of Physical Forces," viz., that " it 

 requires no great stretch of imagination to conceive light 

 and electricity as motions, and not as things moving," in 

 which the writer clearly contrasts the undulatory and cor- 

 puscular theories, as commonly understood, linough has 

 nowbeen advanced to show that the reviewer need not look 

 very far from home for a conspicuous example of that 

 which he has so freely attributed to the unhappy physic- 

 ists, namely that they do but " darken counsel by words 

 without knowledge " (p. 23). 



The equivalence of dynamic and thermic energy is the 

 only one that has as yet been determined quantitatively. 

 He must be a bold man who denies that the sun shines at 

 noonday ; and scarcely less audacious is the assertion of 

 the reviewer that the experiments of Dr. Joule do not con- 

 firm this equivalence. Dr. Joule conducted four distinct 

 series of experiments, three series on the amount of thermic 

 energy produced by molecular friction in stirring respec- 

 tively water, oil, and mercury ; the fourth, on that produced 

 by the friction of two iron surfaces against each other. The 

 four numerical results accorded very nearly, and after 

 assigning to each result its weight, according to its esti- 

 mated liability to error, he deduced the mean value of 772 

 foot-pounds as the dynamic equivalent of thermic energy.t 

 In the metrical system, in which the units of quantity are 

 one kilogramme, one metre, and one degree in the centi- 

 grade scale, the above equivalent is represented by 424 

 dynamic units, which, for brevity's sake, we may as well 

 agree with the French in calling "dynams." 



The reviewer, in ignorance probably of the amount of 

 labour bestowed on this subject, seems to imagine that by 

 ignoring Dr. Joule's results, he has demolished the basis 

 of thermodynamics ; but if so, he is grievously mistaken. 

 It is a remarkable and unprecedented confirmation of 

 this theory, that the numerical results arrived at by three 

 distinct methods of investigation, in the hands of as many 

 independent physicists, should be found to agree within 

 very narrow limits of error. 



It has been found by experiment that aless amount of heat 

 is required to raise a gas maintained at a constant vohune 

 one degree of temperature, than when the gas is allowed 

 to expand under a constant pressnyt'. Suppose, for ex- 

 ample, that the gas be enclosed in a vertical cylinder under 

 a piston of 100 square inches area, the atmospheric pres- 

 sure on this piston will be 1,500 lbs., and the raising this 

 piston is equivalent to raising a weight of that amount. 

 Dr. J. R. Mayer, assuming that the difierence in the quan- 

 tities of heat in the two cases above mentioned is equiva- 

 lent to the work done by the expanding gas, proceeds to 

 determine the numerical value of these equivalent quanti- 





* Lecture on Force and Energy, delivered at the Roy.il Institution, Medi- 

 cal Timesniid Gazette, July S. 1871. 



+ For the s.iWe of those readers 

 ject, we may state that a foot-pound is th( 

 weight of one pound in descending through the ver 

 in other words, the amount necessary to raise one 

 numerical equivalent here given means that 772 dy 

 to the amount of thermic energy required to raisi 



dy familiar with this sub- 

 of energy acquired by a 

 tical space of one foct, or, 

 pound one foot ; and the 

 namic units are equivalent 

 temperalu 



1 temperature of the 



: degn 



