August 25, 1905.] 



SCIENCE. 



231 



an ideal mechanical system or model, 

 which is intended to represent in its lead- 

 ing: 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 configura- 

 tions, together with the circumstances 

 which will cause transitions from one to 

 the other. Even when the working model 

 has been successfully 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 oc- 

 cupied by an account of various investiga- 

 tions which will illustrate the principles 

 and methods which I have now explained 

 in general terms. 



The fascinating idea that matter of all 

 kinds has a common substratum is of re- 

 mote antiquity. In the middle ages the 

 alchemists, inspired by this idea, conceived 

 the possibility of transforming the baser 

 metals into gold. The sole difficulty 

 seemed to them the discovery of an appro- 

 priate series of chemical operations. AVe 

 now know that they were 'always indefi- 

 nitely far from the goal of their search, 

 yet we must accord to them the honor of 

 having been the pioneers of modern chem- 

 istry. 



The object of alchemy, as stated in 

 modern language, was to break up or dis- 

 sociate the atoms of one chemical element 

 into its component parts, and afterwards 

 to reunite them into atoms of gold. Al- 

 though even the dissociative stage of the 



alchemistic problem still lies far beyond 

 the power of the chemist, yet modern re- 

 searches 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 trans'formation of elements. In- 

 deed, ■ in the complex changes which are 

 found to occur spontaneously in uranium, 

 radium and the allied metals we are prob- 

 ably watching a spontaneous dissociation 

 and transmutation of elements. 



Nature 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 ac- 

 cepted by the chemist as an ultimate fact, 

 and indeed the very name of atom, or that 

 which can not be cut, was given to what 

 was supposed to be the final indivisible 

 portion of matter. The chemist thus pro- 

 ceeded in much the same way as the biol- 

 ogist who, in discussing evolution, accepts 

 the species as his working unit. Accord- 

 ingly, until recently the chemist discussed 

 working models of matter of atomic struc- 

 ture, and the vast edifice of modern chem- 

 istry has been built with atomic bricks. 



But within the last few years the elec- 

 trical researches of Lenard, Rontgen, Bec- 

 querel, 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 constitution of matter. 

 Amongst all these fertile investigators it 

 seems to me that Thomson stands pre- 

 eminent, because it is principally through 

 him that we are to-day in a. better position 

 for picturing the structure of an atom 

 than Avas ever the case before.^ 



