August 23, 1888] 



NATURE 



389 



much larger number of species than was formerly suspected, and 

 that the modification is invariably in the direction of protection. 

 The experiments prove also that the stimulus prompting the 

 colour change is given by the colour of the surroundings, but the 

 precise means by which the stimulus is conveyed to the pigment- 

 secreting cells has not yet been made out. This part of the 

 work is no doubt the most difficult to deal with from the 

 experimental side, but any objection to the Darwinian explana- 

 tion which may be urged from the point of view of our ignorance 

 of the nature of this correlation between an external stimulus and 

 the power of secreting a particular colour applies with equal or 

 greater force to the theory of "direct action " upon which so much 

 stress is laid by the new Lamarckian school. The difficulty in the 

 way of completing the explanation of this kind of action is 

 of precisely the same nature as that which meets us when we 

 attempt to explain the power of colour adaptability in a frog 

 or fish as depending upon a colour stimulus, which in these cases 

 is known to be conveyed through the eye. All that is con- 

 tended for is that the power of adaptation has been conferred by 

 natural selection, an agency capable of dealing with complex 

 physiological relationships in precisely the same way that it 

 deals with all other kinds of variations. In these cases of 

 variable protective colouring we are concerned with the origin 

 of the initial variations only in the same manner that we are 

 concerned with their origin in ordinary cases of protective 

 resemblance. Why the colour variability should always be 

 restricted to the limits of protective shades is perfectly intelli- 

 gible from the purely Darwinian stand-point, but is, as it appears 

 to me, absolutely devoid of meaning if we accept the theory of 

 " direct action." R. Meldola. 



August 18. 



MODERN VIEWS OF ELECTRICITY.' 

 Part IV.— Radiation. 



IX. 



SO far as we have been able to understand and explain 

 electrical phenomena, it has been by assuming the 

 existence of a medium endowed with certain mechanical 

 or quasi-mtc\\?ir\\ca.\ properties, such as mobility, incom- 

 pressibility or infinite elasticity of volume, combined with 

 a certain amount of plasticity or finite elasticity of shape. 

 We also imagined the medium as composed of two 

 opposite constituents, which we called positive and negative 

 electricity respectively, and which were connected in such 

 a way that whatever one did the other tended to do the 

 precise opposite. Further, we were led to endow each of 

 these constituents with a certain amount of inertia, and 

 we recognized something of the nature of friction between 

 each constituent and ordinary matter. 

 Broadly speaking we may say— 



(1) That friction makes itself conspicuous in the 

 discussion of current-electricity or the properties of 

 conductors, and that the laws of it are summarized in the 

 statement known by the name of Ohm, viz. that the 

 current through a given conductor is proportional to the 

 force that drives it, or that the opposition force exerted 

 by a conductor upon a current is simply proportional to 

 the strength of that current. 



(2) That elasticity is recognized as necessary when 

 studying the facts of electrostatics or the properties of 

 insulators — electric displacement and recoil, or charge 

 and discharge : the laws having been studied by Faraday, 

 and the relative pliability (or shearability if there were 

 such a word) of the medium in different substances being 

 measured and stated in terms of that of air as their specific 

 inductive capacity, K. 



(3) That inertia is brought into prominence by the 

 facts of magnetism, studied chiefly perhaps by Thomson, 

 who has called the relative density of the medium in 

 different substances their magnetic permeability or mag- 

 netic inductive capacity ; the ratio of its value for any 

 substance to its value for common air being called p. 



(4) That the dottbleness of constitution of the medium 



1 Continued from vol. xxxvii. p. 368. 



— its being composed of two precisely opposite entities — 

 is suggested by the facts of electrolysis, by the absence of 

 mechanical momentum in currents and magnets, and by 

 the difficulty of otherwise conceiving a medium endowed 

 with rigidity which yet is perfectly fluid to masses of 

 matter moving through it. 



With the hypothesis of doubleness of constitution 

 this difficulty disappears. The ether as a whole may be 

 perfectly fluid and allow bodies to pass through it with- 

 out resistance, while its two components may be 

 elastically attached together and may resist any forces 

 tending to separate them with any required rigidity. It 

 is like the difference between passing one's hand through 

 water, and chemically decomposing it ; it is like the 

 difference between waving a piece of canvas about, and 

 tearing it into its constituent threads. 



To put the matter boldly and baldly : we are familiar 

 with the conceptions of matter and of ether, and it is 

 known that the two things react on each other in some 

 way, so that although matter appears to move freely through 

 a free portion of the ether, yet another portion appears 

 to move with matter as if bound to it. This mode of 

 regarding the facts is as old as Fresnel. We now proceed 

 a step further, and analyze the ether into two constituents 

 — two equal opposite constituents — each endowed with 

 inertia, and each connected to the other by elastic ties : 

 ties which the presence of gross matter in general weak- 

 ens and in some cases dissolves. The two constituents 

 are called positive and negative electricity respectively, 

 and of these two electricities we imagine the ether to be 

 composed. The tie between them is dissolved in metals, 

 it is relaxed or made less rigid in ordinary insulators. 

 The specific inductive capacity of a substance means the 

 reciprocal of the rigidity of its doubly constituted ether. 



Let us call this rigidity k, so that k = - . 



K 



The neighbourhood of gross matter seems also to 

 render ether more dense. It is difficult to suppose that 

 it can really condense an incompressible fluid, but it may 

 load it or otherwise modify it so as to produce the effect 

 of increased density. In iron this density reaches its 

 highest known value, and in all substances the density or 

 inertia per unit volume of their ether may be denoted by 

 fx, and called their magnetic permeability. 



Let it be understood what we are doing. In Part I. we 

 discussed effects very analogous to those which would be 

 produced by an elastic incompressible medium (roughly 

 like india-rubber or jelly). In Parts II. and III. we dis- 

 cussed effects suggesting, and more or less necessitating, 

 the idea of a property of the medium very analogous to 

 inertia ; and we were also led to postulate a doubleness 

 of constitution for the medium, so that shearing strains 

 may go on in it and yet it be perfectly fluid as a whole. We 

 are now pushing these analogies and ideas into greater 

 definiteness and baldness of statement. We already 

 know of a continuous incompressible fluid filling all 

 space, and we call it the ether. Let us suppose that it is 

 composed of, and by electromotive force analyzable into, 

 two constituents ; let these constituents cling together 

 with a certain tenacity, so that the medium shall have an 

 electromotive elasticity, though mechanically quite fluid ; 

 and let each constituent possess inertia, or something so 

 like inertia as to produce similar effects. Making this hypo- 

 thesis, electrical effects are to a certain extent explained. 

 Not ultimately indeed — few things can be explained ulti- 

 mately — not even as ultimately as could be wished ; for the 

 nature of the connection between the two constituents of 

 the ether and between the ether and gross matter — the 

 nature of the force, that is, and the nature of the inertia — 

 remains untouched. This is a limitation to be clearly 

 admitted; but if that were the only one — if all else in the 

 hypothesis were true — we should do well, and a distinct 

 step would have been gained. It is hardly to be hoped that 

 this is so — hardly to be expected that the bald statement 



