240 SCIENCE AND THE HUMAN MIND 



While all this new knowledge has expanded amaz- 

 ingly the bounds of science as conceived thirty years 

 The Individual a g> in many ways it has but served to 

 Atom. confirm and give confidence in some of 

 the fundamental concepts in which that science was 

 expressed. In especial, the atomic theory has come 

 triumphantly out of the ordeal, and has proved its 

 continued value as an interpreter of nature at the 

 present stage of the advancement of learning. 



Till recent years, the theory that matter consists 

 of an enormous number of discrete molecules and 

 atoms rested on physical and chemical evidence of an 

 indirect character. The effects of the individual 

 molecule or atom seemed for ever beyond the power 

 of direct perception. We could but infer them from 

 the effects of many millions acting together, as the 

 scout in an airship at a high altitude might infer the 

 presence of individual soldiers from the movement 

 of the columns of an army on the march. Even 

 after the striking success which attended the applica- 

 tion of the atomic theory to the co-ordination of the 

 phenomena of electric conduction in liquids and gases 

 and of radio-activity, there were not wanting sceptics 

 who held that individual atoms must remain purely 

 conceptual entities, removed from the possibility of 

 direct sense-perception. 



But single atoms of helium, shot off by radium as 

 a rays, have been revealed in two ways. Each 

 atomic projectile produces a long train of electric ions 

 as it passes through a gas before its energy is exhausted, 

 perhaps by knocking loose corpuscles out of the mole- 

 cules it encounters in its path. These ions have two 

 effects. They make the gas a conductor of electricity, 



