68 LEAKAGE IN ULTRA-VIOLET LIGHT [CH. vi. 



greater than it is even for an atom of hydrogen, 

 weighed down and burdened as that is with a mass 

 of inert material, and subject only to the very same 

 propulsive force. , i . 



Think of the mobility of a particle which experienced 

 the usual gravitation pull and had only 10 1 00 of the 

 corresponding mass to carry. Such a mobile particle 

 as that would drop under the influence of gravity, not 

 16 feet in the first second, as everything we know 

 does near the surface of the earth, but 16,000 feet, 

 or about three miles ; and would in one second 

 acquire under gravity a velocity of six miles per 

 second ; enough almost to carry it out of the range 

 of the earth's attraction altogether, and more than 

 enough to carry it round the world, if fired horizon- 

 tally with such a speed. 



The acceleration to which particles are subject in a 

 vacuum tube is far greater even than this, because 

 there the forces are so prodigious ; gravitation force 

 on ions is almost infinitesimal compared with common 

 electrical force on their charges. Suppose, for in- 

 stance, that they are in a field such as may easily occur 

 in a vacuum tube, of 3,000 volts per centimetre, one- 

 tenth of what ordinary air will stand, or ten electro- 

 static units. The force urging one of these carriers 

 to move is then 10 x 10~ 10 = 10~ 9 dyne; the mass 

 being moved, if it is a whole atom of hydrogen, e.g. 

 if it were a positive carrier in a hydrogen atmosphere, 

 is only 10~ 24 gramme; and accordingly the acceleration 

 it experiences is 10 15 centimetres per second per second, 

 or a billion times g. Whereas if it were the smallest 

 kind of negative carrier, its acceleration would be a 

 thousand times greater still. 



. The velocity acquired in passing over a distance of 

 five centimetres under this force is obtained by finding 



