260 The Outline of Science 



say, at least 600 miles a second, or thereabouts. Otherwise the 

 electron sticks to the first atom it meets." These amazing par- 

 ticles may travel with the enormous velocity of from 10,000 to 

 more than 100,000 miles a second. It was first learned that they 

 are of an electrical nature, because they are bent out of their 

 normal path if a magnet is brought near them. And this fact led 

 to a further discovery: to one of those sensational estimates which 

 the general public is apt to believe to be founded on the most ab- 

 struse speculations. The physicist set up a little chemical screen 

 for the "Beta rays" to hit, and he so arranged his tube that only a 

 narrow sheaf of the rays poured on to the screen. He then drew 

 this sheaf of rays out of its course with a magnet, and he accur- 

 ately measured the shift of the luminous spot on the screen where 

 the rays impinged on it. But when he knows the exact intensity 

 of his magnetic field which he can control as he likes and the 

 amount of deviation it causes, and the mass of the moving par- 

 ticles, he can tell the speed of the moving particles which he thus 

 diverts. These particles were being hurled out of the atoms of 

 radium, or from the negative pole in a vacuum tube, at a speed 

 which, in good conditions, reached nearly the velocity of light, 

 i.e. nearly 186,000 miles a second. 



Their speed has, of course, been confirmed by numbers of 

 experiments ; and another series of experiments enabled physicists 

 to determine the size of the particles. Only one of these need be 

 described, to give the reader an idea how men of science arrived 

 at their more startling results. 



Fog, as most people know, is thick in our great cities because 

 the water-vapour gathers on the particles of dust and smoke that 

 are in the atmosphere. This fact was used as the basis of some 

 beautiful experiments. Artificial fogs were created in little glass 

 tubes, by introducing dust, in various proportions, for supersatu- 

 rated vapour to gather on. In the end it was possible to cause 

 tiny drops of rain, each with a particle of dust at its core, to fall 

 upon a silver mirror and be counted. It was a method of count- 



