ELECTRON THEORY or CORPUS CULAR THEORY. The physical theory that the atoms of bodies are composite systems, consisting in part (at least) of corpuscles of a still higher order of minuteness. The theory has heretofore concerned itself mainly with the study of one special form of corpuscle, which is exceedingly prominent in sub-atomic phenomena, and which appears to constitute a sort of basic and omnipresent structural element in the architecture of material atoms of all kinds, inasmuch as it has identically the same properties in every respect, whatever the kind of matter from which it is obtained. These funda mental corpuscles usually occur in combination with structural elements of other kinds to form the atoms, but they are also. capable of existing in the free state, and they can be isolated by suitable experimental means. Each corpuscle carries a certain definite and constant charge of negative electricity, which is the same for all of them; and many authorities believe that the corpuscles are, in fact, mere isolated, disem bodied electric charges. Whether this be so or not, experimental evidence indicates that the charge associated with a corpuscle has one definite value, and that it is incapable of varia tion. An atom is supposed to consist of a cer tain number of these negative corpuscles, asso ciated in some definite way with a positively electrified nucleus,— there being, normally, just enough of the negative corpuscles present to neutralize the effects of the positive electrifica tion of the nucleus. According to this view, bodies acquire positive charges by losing some of their negative corpuscles, and acquire nega tive charges by picking up additional negatrve corpuscles. If this hypothesis be correct, it is evident that communicating an electric charge to a body is not a continuous process. It is, on the other hand, essentially discontinuous, and consists in adding to the body (or subtracting from it) a number of definite (though exceed ingly small) units of electricity,— being roughly analogous to filling (or emptying) a barrel by means of a bucket, instead of by the use of a hose. Moreover, the belief that the charging process is essentially discontinuous is no longer based upon theory alone, for Millikan has ob tained direct experimental evidence of such discontinuity, in connection with charges com municated to oil drops by friction.
It happens that the investigation of the posi tive nucleus of the atom is more difficult than the investigation of the negative corpuscles that are normally associated with this nucleus,— or perhaps it would be more accurate to say that the experimental methods thus far devised are mainly applicable to the study of the negative corpuscle. Under certain circum
stances atoms can lose positive charges as well as losing or gaining negative ones; but the loss of a positive charge appears to involve a funda mental change of some kind in the nature of the atom. (See MOLECULAR THEORY.) That there is probably an exceedingly important dif ference between positive and negative electric ity is plainly indicated by the fact that no posi tive charge has yet been demonstrably observed in connection with a mass smaller than that of the hydrogen atom, while negative charges (as will presently appear) are known to occur in connection with masses far more minute than this.
The definite charge carried by, the negative corpuscle appears to be identical in magnitude with the charge carried by the hydrogen ion any other monovalent ion), in electrolysis. or this charge Dr. G. Johnstone Stoney, as long ago as 1891, proposed the name "electron" (Lord Kelvin preferred °electrion," but his sug gestion has not been followed) ; and this name has been applied quite generally, in recent years, to the negative corpuscles themselves, instead of being restricted to the electric charges that they bear. Present practice among authoritative i writers, however, is toward the restoration of the word to its original sense as the name of a definite quantity (or unit) of electric ity, and toward the adoption of J. J. Thom son's original name, °corpuscle') or °negative corpuscle," for the actual particle that bears (or' consists of) a charge of one electron of negative electricity. In the present article we shall follow the tendency here noted, and shall call the particle itself a °corpuscle" or °negative corpuscle," and the charge that it bears an °electron" or "negative electron." The corpuscular theory of matter has been developed in many directions, and its bearing upon the varied phenomena of physics has been extensively investigated by both mathematical and experimental methods. To indicate all its bearings and relations would require a volume, and the present article will therefore be de voted to an explanation of the origin of the corpuscular theory, and to an account of the numerical results that have been obtained in studying the mass, charge, speed and size of the negative corpuscle. Further data concern ing the application of the corpuscular theory to specific physical phenomena may be had from the references, given below.