August 17, 1905] 



NA TURE 



371 



does not rest satisfied unless he obtains a quantitative 

 estimate of various causes and effects on the systems of 

 matter which he discusses. Yet there are some problems 

 of physical evolution in which the conditions are so com- 

 plex that the physicist is driven, as is the biologist, to rest 

 satisfied with qualitative rather than quantitative con- 

 clusions. But he is not content with such crude con- 

 clusions except in the last resort, and he generally prefers 

 to proceed by a different method. 



The mathematician mentally constructs an ideal 

 mechanical system or model, which is intended to repre- 

 sent in its leading features the system he wants to ex- 

 amine. It is often a task of the utmost difficulty to 

 devise such a model, and the investigator may perchance 

 unconsciously drop out as unimportant something which 

 is really essential to represent actuality. He next examines 

 the conditions of his ideal system, and determines, if he 

 can, all the possible stable and unstable configurations, 

 together with the circumstances which will cause 

 transitions from one to the other. Even when the work- 

 ing model has been successfullv imagined, this latter task 

 may often overtax the powers of the mathematician. 

 Finally it remains for him to apply his results to actual 

 matter, and to form a judgment of the extent to which 

 it is justifiable to interpret nature by means of his 

 results. 



The remainder of my address will be occupied by an 

 account of various investigations which will illustrate the 

 principles and methods which I have now explained in 

 general terms. 



The fascinating idea that matter of all kinds has ii 

 common substratum is of i emote antiquity. In the 

 Middle Ages the alchemists, inspired bv this idea, con- 

 ceived the possibility of transforming the baser metals 

 into gold. The sole difficulty seemed to them the discovery 

 of an appropriate series of chemical operations. We now 

 know that they were always indefinitely far from the goal 

 of their search, yet we must accord to them the honour 

 of having been the pioneers of modern chemistry. 



The object of alchemy, as stated in modern language, 

 was to break up or dissociate the atoms of one chemical 

 element into its component parts, and afterwards to re- 

 unite them into atoms of gold. Although even the dis- 

 sociative stage of the alchemistic problem still lies far 

 beyond the power of the chemist, yet modern researches 

 seem to furnish a sufficiently clear idea of the structure 

 of atoms to enable us to see what would have to be done 

 to effect a transformation of elements. Indeed, in the 

 complex changes which arc found to occur spontaneously 

 in uranium, radium, and the allied metals we are prob- 

 ably watching a spontaneous dissociation and transmuta- 

 tion of elements. 



Natural selection may seem, at first sight, as remote 

 as the poles asunder from the ideas of the alchemist, yet 

 dissociation and transmutation depend on the instability 

 and regained stability of the atom, and the survival of the 

 stable atom depends on the principle of natural selection. 



Until some ten years ago the essential diversity of the 

 chemical elements was accepted by the chemist as an 

 ultimate fact, and indeed the very name of atom, or that 

 which cannot be cut, was given to what was supposed to 

 be the final indivisible portion of matter. The chemist 

 thus proceeded in much the same way as the biologist who, 

 in discussing evolution, accepts the species as his working 

 unit. Accordingly, until recently the chemist discussed 

 working models of matter of atomic structure, and the vast 

 edifice of modern chemistry has been built with atomic 

 bricks. 



But within the last few years the electrical researches 

 of Lenard, Rontgen, Becquerel, the Curies, of my 

 colleagues Larmor and Thomson, and of a host of others, 

 have shown that the atom is not indivisible, and a flood 

 of light has been thrown thereby on the ultimate con- 

 stitution of matter. Amongst all these fertile investigators 

 it seems to me that Thomson stands preeminent, because 

 it is principally through him that we are to-day in a better 

 position for picturing the structure of an atom than was 

 ever the case before. 



Even if I had the knowledge requisite for a complete 

 exposition of these investigations, the limits of time would 



NO. 1868, VOL. 72] 



compel me to confine myself to those parts of the subject 

 which bear on the constitution and origin of the elements. 



It has been shown, then, that the atom, previously sup- 

 posed to be indivisible, really consists of a large number 

 of component parts. By various convergent lines of ex- 

 periment it has been proved that the simplest of all atoms, 

 namely that of hydrogen, consists of about 800 separate 

 parts ; while the number of parts in the atom of the denser 

 metals must be counted by tens of thousands. These 

 separate parts of the atom have been called corpuscles or 

 electrons, and may be described as particles of negative 

 electricity. It is paradoxical, yet true, that the physicist 

 knows more about these ultra-atomic corpuscles and can 

 more easily count them than is the case with the atoms 

 of which they form the parts. 



The corpuscles, being negatively electrified, repel one 

 another just as the hairs on a person's head mutually 

 repel one another when combed with a vulcanite comb. 

 The mechanism is as yet obscure whereby the mutual re- 

 pulsion of the negative corpuscles is restrained from break- 

 ing up the atom, but a positive electrical charge, or some- 

 thing equivalent thereto, must exist in the atom, so as to 

 prevent disruption. The existence in the atom of this 

 community of negative corpuscles is certain, and we know- 

 further that they are moving with speeds which may in 

 some cases be comparable to the velocity of light, namely, 

 200,000 miles a second. But the mechanism whereby they 

 are held together in a group is hypothetical. 



It is only just a year ago that Thomson suggested, as 

 representing the atom, a mechanical or electrical model 

 the properties of which could be accurately examined by 

 mathematical methods. He would be the first to admit 

 that his model is at most merely a crude representation of 

 actuality, yet he has been able to show that such an atom 

 must possess mechanical and electrical properties which 

 simulate, with what Whetham describes as " almost 

 Satanic exactness," some of the most obscure and yet 

 most fundamental properties of the chemical elements. 

 " Se non k vero, k ben trovato," and we are surely justified 

 in believing that we have the clue which the alchemists 

 sought in vain. 



Thomson's atom consists of a globe charged with positive- 

 electricity, inside which there are some thousand or 

 thousands of corpuscles of negative electricity, revolving^ 

 in regular orbits with great velocities. Since two electrical 

 charges repel one another if they are of the same kind, 

 and attract one another if they are of opposite kinds, the 

 corpuscles mutually repel one another, but all are attracted 

 by the globe containing them. The forces called into play 

 by these electrical interactions are clearly very complicated, 

 and you will not be surprised to learn that Thomson found 

 himself compelled to limit his detailed examination of the 

 model atom to one containing about seventy corpuscles. 

 It is indeed a triumph of mathematical power to have 

 determined the mechanical conditions of such a miniature 

 planetary system as I have described. 



It appears that in general there are definite arrange- 

 ments of the orbits in which the corpuscles must revolve, 

 if they are to be persistent or stable in their motions. 

 But the number of corpuscles in such a community is not: 

 absolutely fixed. It is easy to see that we might add a- 

 minor planet, or indeed half a dozen minor planets, to 

 the solar system without any material derangement of the 

 whole ; but it would not be possible to add a hundred 

 planets with an aggregate mass equal to that of Jupiter 

 without disorganisation of the solar system. So also we 

 might add or subtract from an atom three or four cor- 

 puscles from a system containing a thousand corpuscles 

 moving in regular orbits without any profound derange- 

 ment. As each arrangement of orbits corresponds to the- 

 atom of a distinct element, we may say that the addition 

 or subtraction of a few corpuscles to the atom will not 

 effect a transmutation of elements. An atom which has 

 a deficiencv of its full complement of corpuscles, which it 

 will be remembered are negative, will be positively- 

 electrified, while one with an excess of corpuscles will be 

 negatively electrified. I have referred to the possibility- 

 of a deficiency or excess of corpuscles because it is im- 

 portant in Thomson's theory; but, as it is not involved in 

 the point of view which I wish to take, I will henceforth 

 onlv refer to the normal or average number in any arrange 



