Feb. 5, 1880] 



NATURE 



335 



Hypothesis is a veritable Scylla that captivate-; the imagination 

 and often sends the visionary to destruction, while practice alone 

 is a hard-hearted Charybdis that lures the matter-of-fact prac- 

 tical man to folly and expense. Practice must be tempered with 

 theory to utilize advantageously the great forces of nature, and 

 theory itself must be based on practice, or on facts, to be compre- 

 hensive and acceptable. Hence success is the offspring of the 

 marriage of practice and theory, and, therefore, as the two are 

 so intimately connected, I have determined to steer a middle 

 course to-night to survey the progress of each in our profession, 

 and to show their mutual relationship. 



What is theory ? It is an explanation of the hidden cause of 

 certain effects that are evident to the senses. It is an effort of 

 the imagination to account for operations that are in themselves 

 invisible and insensible, but which result in facts that are observ- 

 able and known. Thus the movements of all those bright bodies 

 by which 



" the floor of heaven 

 Is thick inlaid with patines of bright gold," 



are explained by the theory of gravity. Their appearance, 

 vagaries, and beauties are accounted for by the undulatory theory 

 of light. The warmth that the monarch of them all shed upon 

 this earth countless ages ago, and that is now restored to us in 

 our household fires, is explicable on the molecular theory of heat. 

 The constitution of matter and its various states of solid, 

 liquid, and gas, are completely explained by the atomic 

 theory of Democritus and Dalton, and the modern kinetic 

 theory of gases. 



It is impossible for a practical man who has devoted more than 

 a quarter of a century to the application of electricity to useful 

 purposes, to avoid devoting much contemplation to the nature of 

 the agent which he has to make use of. Is there a member of 

 this society who has not striven to peer into the region of the 

 unknown, who has not speculated on the power he uses, or who 

 has not formed some conception in his mind of the nature of 

 electricity ? Yet it is remarkable that the answer to the question, 

 What is electricity? cannot even now be given with authority. 

 Faraday, our great apostle, whose researches should be every 

 electrician's bible, declined to venture an answer, nor did he 

 ever directly formulate his ideas on the subject, though his 

 1 ublications indicate pretty clearly and with no uncertain sound 

 what they were. Clerk-Maxwell, who, while he overthrew all 

 existing theories, failed to supply their place before he was so 

 untimely removed from us. Sir William Thomson, in his 

 published papers, always carefully eschews the consideration of 

 any physical theory of electricity. The French electricians 

 simply use the one-fluid theory as a convenience of language, 

 while the Germans, as a rule, employ the two-fluid theory 

 merely for mathematical purposes. Hence there is no recognised 

 theory of electricity. Some maintain with Du Fay or with 

 Franklin, that it is a form of matter — a substance ; others, 

 following Faraday and Grove, consider it a form of force — a 

 motion— like heat and light. It must be either one or the other. 

 There is no other category in which to class it. If it is not a 

 form of matter it must be a form of force. The question I 

 propose to discuss is, therefore, Is electricity a form of matter, 

 or is it a form of force ? 



In discussing such a vexed question it is necessary to be very 

 precise in language to avoid any misconception of my meaning, 

 therefore I will define both matter and force in the sense in which 

 I use those terms. Matter is that which can be perceived by the 

 senses, or can be acted upon by force. It is characterised by 

 weight, inertia, and elasticity. Force is that which produces, or 

 tends to produce, the motion of matter. It may be pressure, 

 tension, attraction, repulsion, or anything capable of causing 

 alteration in the natural state of rest or of existing motion of 

 matter. , 



Matter is found in either the solid, liquid, gaseous, or ultra- 

 gaseous state, and it occupies space. It consists of molecules 

 and atoms. The atom is the smallest indivisible part of an 

 element, and a group of atoms of the same or of different 

 elements forms the molecule, which has a definite magnitude and 

 is unalterable in form for each substance. The mass of a 

 substance is the aggregate of the molecules of which it is com- 

 posed. There is no generation or destruction of atoms. The 

 indestructibility of matter is a fixed law in nature. The size of 

 the molecule is approximately known. Sir William Thomson 

 says : — " If we conceive a sphere of water as large as a pea to 

 be magnified to the size of the earth, each molecule being mag- 

 nified to the same extent, the magnified structure would be 



coarser-grained than a heap of small lead shot, but less coarse- 

 grained than a heap of cricket balls." Fifty million molecules 

 ranged in single file would occupy an inch. They are highly 

 elastic, and unless interfered with would move with constant 

 velocity in straight lines. When they can move about freely 

 without interfering with each other's proceedings, we have the 

 ultra-gaseous state of Crookes, a state found only in very high 

 vacua and under certain adventitious circumstances. When they 

 collide and impinge on each other according to the law of the 

 impact of elastic bodies, interfering with each other's path, we 

 have gases as we know them ; when their mean free path is so 

 reduced as to bring them within the sphere of mutual attraction, 

 without too narrowly restricting their play, we have liquids ; 

 when the attraction becomes cohesion and the motion of the 

 molecule is confined to its own sphere, we have solids. The 

 number of molecules in a given volume of gas is known, and 

 their velocity calculated. In hydrogen the velocity at o" Cent, 

 is 6,097 feet per second, the number being io 2 3 per cubic inch. 

 The mean free path of a molecule in air at ordinary pressures is 

 the ten-thousandth part of a millimetre. Besides their constant 

 motion in straight lines the molecules may be set in vibration, 

 rotation, or any other kind of relative motion whatever. 



This is the atomic theory of matter born in the brain of De- 

 mocritus, "the laughing philosopher," 2,300 years ago ; preached 

 by Epicurus in Athens, and taught by Lucretius in Rome before 

 the Christian era ; lying dormant for eighteen centuries, until 

 it was formulated by Dalton in the last century, and removed 

 from the region of pure speculation by Joule, Clausius, Clerk- 

 Maxwell, and Crookes during our days. 



The definition of force shows us that whatever changes or 

 tends to change the motion of matter (or of the molecules of 

 which it is composed), by altering either its direction or its mag- 

 nitude, is a form of force. Thus gravity is a form of force, for it 

 attracts all matter to the centre of the earth, and it is measured 

 by the rate per second at which a body acquires a velocity in this 

 direction when falling freely at a given spot. Heat is a form of 

 force, for it throws the molecules of matter into violent vibration, 

 or it increases the velocity of their motion in straight lines, which 

 thus becomes the measure of its heat or its temperature. Light 

 is a form of force, for it is produced by the undulation of the 

 molecules of matter, and it is transmitted by the undulations of 

 that medium called Ether, which fills all space. 



When we take a given free mass and impress upon it a given 

 force, we throw that mass into motion ; for instance, when we 

 fire a loaded cannon, we have imparted to the ball "energy" 

 and in virtue of the motion of the ball, this energy is called 

 "kinetic." Again, if we lift the ball to a certain height above 

 the earth's surface — say to the top of a tower — and let it remain 

 there, we have again imparted to it " energy," but this time it is 

 called "potential" for it is dormant or resting. In each case 

 the energy possessed by the ball is the exact equivalent of the 

 work done upon it, that is, of the force impressed and the 

 distance through which it has acted. The motion of the ball is 

 readily transferred to the motion of the individual molecules of 

 the ball. When, in the first case adduced, the ball strikes the 

 side of a ship or a target, its kinetic energy is thus converted 

 into light and heat, which is molecular motion ; or, in the second 

 case, when it is allowed to fall, its potential energy is converted 

 into kinetic energy, which again, on coming in contact with the 

 ground, is converted into molecular motion or heat. Energy is 

 always either potential or kinetic, and one of the most remarkable 

 generali-ations of modern days is the grand principle of the con- 

 servation of energy, which implies that the total energy of the 

 universe is a quantity which can neither be increased nor 

 diminished, though it may be transformed into any of the forms 

 of which energy is susceptible. Energy is therefore as inde- 

 structible as matter. All the recent advances in the science of 

 heat have been due to the discovery of this principle, and its 

 application to electricity has gone far to remove that science from 

 the hypothetical state in which it has existed so long. 



My purpose is to contend that electricity is not a form of 

 matter but a form of force, and that all its effects are evident to 

 us in one or other of the several forms of energy characterised 

 by the motions of molecules or of mass. 



It is interesting to trace the historical growth of theories. 

 The uncultivated human intellect cannot soar above its own 

 limited sphere of childish observation. Whatever is mysterious 

 and incomprehensible in nature is attributed to that which is 

 equally mysterious and incomprehensible. Life has ever been 

 of this character, and heat, magnetism, electricity, and many 



