158 THE REALITIES OF MODERN SCIENCE 



paper, was thrown into the swarm. It wouldn't fall 

 very quickly, for it would be buffeted first one way 

 and then another, now up and now down, by the 

 impacts of the various gnats which collided with it. 

 Such a piece of paper might be made so small that 

 only rarely would two or more gnats happen to collide 

 with it simultaneously. The observer while unable 

 to see the gnats would then see this paper moving in 

 space, irregularly and jerkily, as it was jostled by 

 individual gnats. 



Now this is- almost what actually happens in the case 

 of molecules. The human eye is adapted by ages of 

 evolution to see large objects which are of interest to 

 human beings. Even with the aid of the best possible 

 microscope it can never see directly objects smaller 

 than a certain definite size determined by the color of 

 light with which they are illuminated. This size is, 

 however, much larger than the largest known mole- 

 cule. In 1827 a botanist, Robert Brown, observed 

 through the microscope the continual movements 

 of some minute particles which were suspended in the 

 liquid which he was examining. The movements 

 of small bodies due to the impacts of molecules are, 

 therefore, known to-day as "Brownian Movements. " 

 For years no use was made of this discovery, although 

 the bacteriologists learned to distinguish between 

 these movements of small suspended particles and 

 those of the bacteria for which they were searching 

 with microscopes. In recent years many very im- 

 portant ideas as to molecular movements and also as 

 to electronic movements have been obtained by appli- 

 cations of this phenomenon. 



