108 THE PATH OF SCIENCE 



science. That they were due to particles was shown by the 

 fact that they could be deviated by a transverse electrostatic 

 field. Tests on this subject had failed previously because the 

 gas pressure in the tube was too high. In 1897, then, Thom- 

 son finally showed that the cathode rays consist of charged 

 particles and that these particles are very small— about one 

 two-thousandth of the inass of the hydrogen atom. The name 

 electron had already been given to the atom of negative elec- 

 tricity by Johnstone Stoney in 1874, at the time that he put 

 forward his idea of atomistic electricity. It was now realized 

 that the particles of the cathode rays are electrons. 



Another application of this inethod of using electro-mag- 

 netic and electrostatic fields to control a stream of elec- 

 tricity in a vacuum ^vas applied to the positively charged 

 streams that come from the anode. These positive rays can 

 be deviated by a magnetic field and also by an electrical field, 

 but the amount of the deviation is much less than that of the 

 cathode rays because the particles froin the anode are much 

 heavier than those from the cathode. They can be shown to 

 consist of streams of atoms or inolecules. Moreover, such a 

 stream contains a number of different atoms, and since these 

 are of different mass, they will be separated by the magnetic 

 field. This work was done by Thomson and his student 

 F. W. Aston. Later, Aston designed an instrument which 

 he called the mass spectrograph^, in which the positive ray 

 passed through a magnetic field so that the atoms of differ- 

 ent mass were separated, the streams of different atoms being 

 detected either by their record on a photographic film or by 

 the measurement of the ionizing po^ver of the stream when 

 allowed to run into a chamber containing gas of ^vhich the 

 conductivity could be measured. A very important result of 

 Aston's work was the discovery that frequently several atoms 

 of the same element exist having different masses. Thus, in 

 the case of neon, one of the first elements to be investigated, 

 about 90 per cent of the gas consists of atoms having a inass 

 of 20, whereas 10 per cent consists of atoms having a mass 

 of 22 units. Atoms having the same chemical properties but 



