THE PRESENT CONCEPTION OF MATTER ^j 



these were named " isotopes " by Professor Soddy. Lead, 

 for example, is one of them. This substance may be obtained 

 in several different ways ; to the chemist it is always lead, but 

 its atomic weight depends on the way in which it has been 

 derived. 



That there was some intimate connection between the 

 elements was also shown by the expulsion of a particle of 

 helium from many of them, and a particle of hydrogen from 

 some. It is not improbable indeed that the atoms of all the 

 elements are built up from atoms of hydrogen and electrons. 

 Thus the older interpretation of matter, which in its ultimate 

 analysis resolved the universe into the eighty or ninety inde- 

 pendent elements, has given place to a monistic, or at most a 

 dualistic, view, and to some extent physics has invaded the 

 realm of metaphysics. 



So we come to the fundamental questions, What are the 

 constituents of an atom ? What are the electrons ? How 

 many are there in the atom ? How is the atom built up from 

 them ? 



The atoms of any particular element are the smallest par- 

 ticles of it that can exist, far too minute for us ever to hope to 

 see. A hundred million of them, placed end to end, might 

 stretch across a penny. They have often been regarded as 

 hard spheres not unlike tiny balls of steel, and in many ways 

 they behave as though they were. They are certainly very 

 difficult to penetrate, and their outer surface alone is touched 

 by any ordinary experiment of a chemical or optical kind. The 

 only particles that can reach their inner parts are those thrown 

 off by the radio-active substances. 



Radio-activity has been so well summarised by C, G. Darwin 

 that we can do no better than refer to his description of the 

 phenomenon. It is a series of transformations of a substance 

 through a number of states, in each of which the temporary 

 substance formed has all the characteristics associated with 

 the word " element." The radio-active elements, indeed, 

 differ from the others only in the fact that in addition to their 

 ordinary chemical and physical properties they are also con- 

 tinuously transmuting themselves spontaneously into other 

 elements. During the transformations there are given off 

 three typical kinds of radiation known as the a, /S, and 7 rays 

 respectively. 



The a particle is an atom of helium having a charge of 

 positive electricity twice as great as that of an electron, so that 

 it is sometimes regarded as having two positive charges each 

 equal to that of an electron, but opposite to it in sign. The 

 latter point of view obviates the necessity of thinking of charges 

 of different magnitudes. The velocity of the particle varies 



