CONTEMPORARY ADVANCES IN PHYSICS 629 



It seems unlikely; for the very special wonderment and admiration, 

 which the first-named change would evoke from a scientist of today 

 were it achieved before him, would arise out of a wisdom denied alike 

 to Greek and Florentine. Only the modern can know how much it 

 would differ, in what particular way it would transcend all that has 

 gone before. 



For his power of appreciation, this modern onlooker would have 

 two sciences to thank. These are the chemistry of the nineteenth and 

 the eighteenth century, with its uncounted and uncountable attempts 

 to analyze and synthetize and convert and transform, which led to the 

 eventual conclusion that underneath the endless and changeable var- 

 iety of visible matter there are certain substances which can neither be 

 synthetized nor analyzed nor converted one into another; and the 

 physics of the twentieth, which penetrated deep into the atoms of these 

 unalterable substances, and there discovered the recondite and all-but- 

 unassailable part, in which the character of each element is conserved. 

 The former proved that all known chemical changes are made by 

 combining these atoms or tearing them apart; the latter showed that 

 what happens in every such event may be imagined as a rearrangement 

 of flocks of electrons, which form the outer part of every atom. But 

 chemistry further proved that such a change as that of silver to copper, 

 or of mercury to gold, must of necessity involve something far more 

 radical— something which the physics of alpha-particles and X-rays 

 eventually made clear: in the atom there is an innermost nucleus, the 

 centre of attraction whereby the electron-flocks are held together: this 

 it is which must be reached and altered, if one element is to be trans- 

 formed into another. 



This statement, being as it is a description of the geography of the 

 atom— perhaps I should say, a description of its astronomy, for these 

 ultimate particles of matter are to be likened to a solar system rather 

 than the earth — requires to be proved by exploration. The explorers 

 sent out for this purpose are alpha-particles— corpuscles which are 

 recognizable as such, for if they strike against a fluorescent screen 

 each makes its separate luminous splash. Less than IQ-'- of a centi- 

 meter in diameter, they are small enough to penetrate the electron- 

 flocks of the atoms, which are spread over spaces tens of thousands of 

 times as wide. The electrons near which they pass deviate them 

 but little, being of less than one seven-thousandth their mass. En- 

 dowed with energy which may be as great as an electron could acquire 

 from a potential-rise of eight million volts, they are able to approach 

 the positive portions of the atomic structure though positively charged 

 themselves. They are, in fact, extremely well fitted for the task of 



