T 2 8 TEE POPULAR SCIENCE MONTELY 



radioactive matter, or of inducing similar effects in ordinary matter, 

 does not at present seem at all promising. 



Structure of the Atom 



We have seen that in recent years a number of methods have been 

 devised for determining with precision the actual weight of any atom 

 of matter. If it be assumed that in the solid state the atoms, or mole- 

 cules, of matter are in close contact, it is a simple matter to deduce the 

 diameter of the atom. This varies slightly for different atoms, but on 

 an average comes out to be about one hundred-millionth of a centimeter. 

 It is necessary, however, to be cautious in speaking of the diameter of 

 the atom. The term " diameter of the sphere of action " of the atom 

 is preferable, for it is not at all certain that the actual atomic structure 

 is nearly so extensive as the region through which the atomic forces are 

 appreciable. 



Even before the discovery of the electron, the general idea had been 

 suggested that the atom was an electrical structure composed of nega- 

 tively and positively charged particles held in equilibrium by electrical 

 forces. Such ideas had been proposed and developed by Larmor and 

 Lorentz in order to explain the electrical and optical properties of the 

 atom. The proof that the negative electron was an independent unit 

 of the structure of the atom gave a great impetus to the formation of 

 more concrete ideas on atomic structure. There was one important 

 difficulty, however, that arose at the outset. While negative electricity 

 had been shown to exist in independent units of very small apparent 

 mass, the corresponding unit of positive electricity was never found 

 associated with a mass less than the atom of hydrogen. All attempts to 

 show the existence of a positive electron of small mass, which is a 

 counterpart of the negative electron, have resulted in failure, and it 

 seems doubtful whether such a positive electron exists. The role played 

 by positive electricity in the atom was thus a matter of conjecture. In 

 a paper called " iEpinus Atomized," the late Lord Kelvin considered an 

 atom to consist of a uniform sphere of positive electrification, through- 

 out which negative electricity was distributed in the form of discrete 

 electrons. In order to make such an atom electrically neutral, it is, of 

 course, necessary that the positive charge should be equal and opposite 

 to the charge carried by the electrons. This idea of the structure of 

 the atom was taken up and developed with great mathematical skill by 

 Sir J. J. Thomson. He investigated the constitution of atoms contain- 

 ing different numbers of electrons, and showed that such model atoms 

 possessed properties very similar to those shown by the actual atoms. 

 The Thomson atom proved for many years very useful in giving a con- 

 crete idea of the possible structure of the atom, and had the great 

 advantage of being amenable to calculation. 



The rapid advance of science in the last decade has provided us 



