336 



NATURE 



[Feb. 5, 1880 



other unaccountable physical phenomena, have each in their turn 

 been supposed to be causes of life. Even now there are those 

 who would attribute exceptional and peculiar phenomena to 

 spiritual agencies. 



Heat was thought by the Greeks to be an animal that bit. It 

 was then for many centuries thought to be a fluid which, 

 entering into bodies, like mercury, made them swell, and this 

 idea existed until this generation, when Rumford showed it to 

 be a kind of motion, and Joule made it a quantitative form of 

 energy. 



Thales of Miletus thought that the magnet was endowed 

 with a sort of immaterial spirit, and to possess a species of 

 animation. The Greeks knew also that rubbed amber attracted 

 bits of straw, and supposed it to be endowed with life. Even 

 Boyle, as late as 1675, imagined it to emit a sort of glutinous 

 effluvium which laid hold of small bodies and pulled them 

 towards the excited body. Du Fay in 1733 conceived the double 

 fluid theory, and Franklin in 1747 invented the single fluid 

 theory. Cavendish in 1 771 supplied some of the deficiencies of 

 Franklin's theory, but it was Faraday who first exploded the 

 fluid notion and originated the molecular theory of electricity, 

 while Grove boldly classed electricity with light and heat .as 

 correlated forces and mere modes of motion. 



Light was thought by the Platonists to be the consequence of 

 something emitted from the eye meeting with certain emanations 

 from the surface of things, but no theory of light properly so- 

 called was attempted until Newton produced his celebrated 

 corpuscular theory in 1670, which has lasted until the present 

 day. Even as late as 1816 Faraday himself said — " The con- 

 clusion that is now generally received appears to be, that light 

 consists of minute atoms of matter of an octahedral form, 

 possessing polarity, and varying in size or in velocity." l 

 Although Huyghens in Newton's own time conceived the undu- 

 latory theory, the superior authority of the great Engli-h 

 philosopher overshadowed the lesser light, and it was not until 

 Young and Fresnel at the commencement of this century took 

 the matter up, that the present theory of light took firm root. 



Thus we see that all these sciences have passed through the 

 same stages of mystery and fancy, and it is only within the present 

 generation that they have emerged from the mythical to the 

 natural, from mere hypothesis to true theory. Hypothesis is an 

 imaginary explanation of the cause of certain phenomena which 

 remains to be shown probable or to be proved true. Theory is 

 this supposition when it has been shown to be highly probable 

 and all known facts are in agreement with its truth. 



A theory, therefore, to be valid and true, must agree with 

 every observed fact ; it must not conflict with natural laws ; 

 it must suggest new experience, and it should leal to further 

 developments. A theory is absurd if it supposes an agent to act 

 in a manner unknown in all other cases. The fluid theories of 

 electricity are merely descriptive, they do not agree with every 

 observed fact ; they have never prompted the invention of a single 

 new experiment, or led to any development. They suppose an 

 agent unknown in other cases and opposed to natural laws. In- 

 complete theories die a natural death : thus Descartes' vortices, 

 Newton's corpuscular theory of light, the fluid theory of heat, 

 Stahl's phlogiston, Nature abhorring a vacuum, have all disap- 

 peared, while complete theories, such as that of gravity, the laws 

 of motion, the conservation of energy, the undulatory theory of 

 light, not only remain, but suggest new fields of inquiry, open 

 out fresh pastures, carry truth and conviction with theui, and 

 have led to the most wonderful predictions. The fluid 

 theories of electricity are certainly incomplete, and they deserve 

 a speedy interment. We have to assume the existence of two 

 substances of opposite qualities which mutually annihilate each 

 other on combination — a self-evident absurdity, for the concep- 

 tion of matter involves indestructibility. Franklin imagined his 

 one fluid to be an element of glass ; remove electricity, and gla>s 

 would lose its virtues and properties, and thus glass was to give 

 out its electricity for ever and a day, without loss of weight or 

 sensible diminution. It was to be devoid of dimensions, inertia, 

 weight, and elasticity, and is therefore outside the pale of our 

 definition. 



Electricity is therefore not a form of matter. Hence, 

 according to our reasoning, it must be a form of force. 



But can we not prove that it is a form of force ? Certainly. 

 Let us first argue from analogy. We know that sound, heat, 

 and light are modes of motion ; in what respects does electricity 

 agree with these forms of force? 



1 " Life," vol. i. p. 216. 



The fundamental law of electrostatics is that two bodie s 

 charged with opposite electricities attract each other with a force 

 dependent on the square of the distance separating them. What- 

 ever influence or power spreads from a point and expands 

 uniformly through space varies in intensity as the square of the 

 distance for the area over which it is spread increases as the 

 square of the radius. This is the case with gravity, light, sound, 

 and heat, which are known forms of force. It is also the case 

 with electricity and magnetism, which ought therefore to be 

 similar forms of force. 



If we regard the velocity of transmission of certain electrical 

 disturbances through space, we have every reason to believe that 

 it is the same as that of radiant heat and light. In 1859 two 

 observers in different parts of the country (Messrs. Carrington 

 and Hodgson) saw simultaneously a bright spot break out on the 

 face of the sun, whose duration was only five minutes. Exactly 

 at this time the magnetic needles at Kew were jerked, and the tele- 

 graph wires all over the world were disturbed. Telegraphi-ts were 

 shocked, and an apparatus in Norway was set on fire. Auroras 

 followed, and all the effects of powerful magnetic storms. More- 

 over, the periods of sun-spots, earth currents, and magnetic storms 

 follow the same cycle of about eleven years. Dr. Hopkinson has 

 shown that this electric disturbance through space is as mechani- 

 cal as its action through short distances, and is therefore iden- 

 tical with the ordinary strains of elastic matter subject to 

 distortion by mechanical force. But Clerk-Maxwell has gone 

 beyond this, and has shown that the velocity of light is identical 

 with that of the propagation of electrical disturbances through 

 space as well as through air and other transparent media. 

 Hence, as light is admitted to be a mode of motion identical 

 with radiant heat, electricity must be of the same category. 



There is such a remarkable analogy between the conductivity 

 of the different metals for heat and for electricity — indeed, there 

 is every reason to believe that if the metals were pure, the order 

 and ratio of conductivity would be identical — that it is impossible 

 to resist the conclusion that the mode of transmission in each 

 case is the same. Mr. Chandler Roberts, who, using Prof. 

 Hughes' beautiful induction-balance, showed, by experiments or» 

 a comprehensive series of alloys, that the curves indicating the 

 induction-balance effect closely resemble their curves of electrical 

 resistance. He was also able to demonstrate that the induction- 

 balance curve of the copper-tin alloys is almost identical with 

 the curve of the conductivity of heat : — a conclusion of much 

 interest ; and he pointed out that we might look with confidence 

 to being able to ascertain, by the aid of the induction-balance, 

 whether the relation between the conductivity of heat and 1 

 electricity is really as simple as it has hitherto been sop] 

 to be. Moreover, when a wire conveys a current of electricity 

 it is warmed, as the strength of current is increased it 

 is heated and eventually rendered incandescent. The ultimate 

 form which every electric current takes is heat. The wire of 

 every telegraph is warmed in proportion to the currents it 

 transmits. Joule showed that when this heat is produced 

 by a current generated in a battery by chemical force, its 

 amount is exactly equivalent to that which would have been 

 evolved by the direct combination of the atoms. The conducting 

 power of all bodies is affected by heat, and some even, like 

 selenium, by light. Hence, as we know that in the case of heat 

 and light conduction is molecular vibration, we reasonably con- 

 clude that it is the same with electricity. In fact, it is im- 

 possible to account for these phenomena except on the assumpri m 

 of the motion of the molecules. 



The magnificent researches of Dr. Warren de la Rue and Dr. 

 Hugo Mviller on the electric discharge with the 11,000 cells of 

 chloride of silvery battery that the former philosopher has 

 provided himself with in his celebrated laboratory, have shown 

 indisputably that the discharge in air or in gases under various 

 pressures is a function of the molecules filling the space through 

 which the discharge occurs. In fact, the resistance of the 

 discharge between parallel flat surfaces is as the number of 

 molecules intervening between them ; and they show that during 

 electrical discharge in a gas there is a sudden and considerable 

 ] ressure produced by a projection of the molecules against the 

 sides of the containing vessel distinct from that caused by heat, 

 and unquestionably due to the molecular action of electrification. 

 The long- continued and patient researches which these eminent 

 physicists are carrying out prove beyond doubt that electrical 

 discharge is simply molecular disturbance. In reality, the fact 

 that no discharge occurs through a perfect vacuum is a crucial 

 proof of the molecular theory. 



