148 PHYSICAL SCIENCE 



elements, progressively known to the chemist, 

 seemed to be essentially different in kind, though 

 certain likenesses between them, and periodic rela- 

 tions between their properties and masses, vaguely 

 pointed to a common origin. Now, after a hundred 

 years, the atom yields place to Thomson's corpuscle 

 as the ultimate known particle of matter ; while 

 the phenomena of radio-activity, as w^e shall see 

 hereafter, have shaken the belief in the immuta- 

 bility of the elements, and are leading to a new- 

 faith in their transmutation. 



Speculation, it is true, from the days of 

 Democritus to those of Sir William Crookes, 

 has been busy with imaginings anent ultimate 

 particles, which should be common to all types of 

 matter, and should compose the different elements 

 by differences in their number or arrangement. 

 But Professor Thomson has not followed the facile 

 and barren paths of speculation. He has first 

 found the particles, and has weighed and timed 

 them before theorising on their origin and destiny. 



We are now in a position to estimate the 

 importance of the experiments which have shown 

 that the mass of the corpuscle is independent both 

 of the nature of the gas in w^hich it is found, and 

 also of the material of the electrode used in pro- 

 ducing it. Not only must we conceive atoms to 

 contain these more minute particles, but it is 

 necessary to suppose that in all atoms, whatever 

 be their nature, these particles are similar. The 

 dream of an ultimate particle, common to all kinds 

 of matter, has thus at length come true. 



The relation between the corpuscles and the 

 electric charges associated with them must next be 

 considered. These isolated particles have never 



