According to present views, all negative cor puscles are precisely alike, no matter from what source they come. The same is be lieved to be true of all positive corpuscles; but the positive corpuscle is believed to be differ ently constituted from the negative corpuscle — notably in respect to size and mass. ,There is also a marked tendency toward the belief that molecules contain nothing else in addition to these positive and negative corpuscles. If this conception be correct, we are to think of mat ter as being composed solely of electric cor puscles, the differences between the chemical elements being merely architectural—one differing from another only in the number and arrangement of the corpuscles of which the atoms consist.
We do not know the precise way in which the atom is put together, but certain funda mental facts in relation to its structure are becoming reasonably clear. In the early days of the electrical theory, Lord Kelvin sug gested that the atom might consist of a spheri cal charge or mass of positive electricity of considerable magnitude, with many negative corpuscles oscillating or revolving within it. If the positive charge were distributed uni formly through the sphere, it would be easy to imagine a state of things in which the oscil lations or revolutions of the negative corpuscles would be performed in certain definite periods and we might thereby hope to account for spec tral lines that highly-heated substances show under suitable conditions. Later investigations have indicated, however, that this conception of the atom is hardly tenable. For example, it is now considered to be quite certain that the positive electricity in the atom is limited to an exceedingly small bulk, instead of occupying a considerable part of the total volume.
It is not possible to review all the theories of molecular structure that have been proposed and considered, nor would it be desirable to do so. because many of them that were .regarded with considerable favor only a few years ago have now been definitely discarded on account of their demonstrated inadequacy. Something must be said, however, of the views that are at present in favor and which have been tested well enough to indicate that they are probably steps in the right direction.
It has long been known that the apparent mass of a body is larger when the ,body is electrically charged than it is when there is no such charge present. The difference is too small to be observed in bodies of ordinary size, but the theory of electricity indicates that if the charge is concentrated in an exceedingly small volume, the apparent mass due to electri fication may become very significant and im portant. Physicists now incline strongly to the view, in fact, that there is no such thing as °real mass" in the old sense of this expres sion, but that the mass of a body is due exclu sively to the presence, within the molecules of the body, of the highly-concentrated electric charges that constitute the corpuscles of which these molescules are composed. (For discussion of electrical mass, see ELECTRON THBDRY).
The mass of a stationary or slowly-moving negative corpuscle has been shown to be equal to the one thousand eight hundred and forty fifth part of the mass of a hydrogen atom. It was thought probable, a few years ago, that we should, therefore, have to assume that the hydrogen atom contains 1,845 of these corpuscles, if we are to explain its mass on the electrical basis. It is now believed, how ever, that the greater part of the mass of the atom is resident in the positive corpuscles that it contains, and if this be true it is no longer necessary to assume that the atom has the highly complicated structure that would be implied by the presence within it, in some form of stable arrangement, of nearly 2,000 negative corpuscles, even in the simplest case.
It appears probable, at the present time that an atom consists of a certain number of centrally-located positive electrons, perhaps as sociated with an unknown number of negative corpuscles and certainly surrounded by other negative , corpuscles which are in a state of rapid motion and which are related to the cen tral nucleus in some such way as the planets of the solar system are related to the sun. Rutherford showed that the number of free positive electrons in a gold atom can be de termined by observing the deflection experi enced by alpha-particles (see RADIOACTIVITY) when these particles are passed through gold foil of known thickness. The experiment was performed in Rutherford's laboratory (Philo sophical Magazine, 1913, p. 604) and was re• peated with foils composed of a number of other metals also, and it was found that, for the metals that were used, the number of free positive electrons in one atom was in each case approximately equal to half the atomic weight of the metal. This result harmonizes with the conclusions reached by Barkla from experi ments of a wholly different kind; and Moseley, by his wonderful researches on the wave-lengths of X-rays, proved, beyond apparent question, that the number of free positive electrons in the nucleus of an atom is equal to the °atomic number" of the element to which the atom be longs — that is, to the number that the element bears when all the elements are arranged in accordance with the so-called periodic law. The results obtained by the methods of Ruther ford and Barkla are only rough and approxi mate; but they substantiate Moseley's conclu sions and • as 3,foselev's results are absolutely definite and extraordinarily well founded, we may conclude that we now know, without much doubt, just how many free positive electrons the nucleus of every atom contains. The hydrogen atom for example, contains 1; carbon atom, 6; chlorine atom, 17; iron atom, 26; gold atom, 79, and uranium, 92. We do not known how these central positive corpuscles are held to gether, but we infer that when an atom has no free electric charge, it contains, distributed through its outer regions, freely-moving nega tive corpuscles equal in number to the free positive electrons in the nucleus.