2 DISCOVERY AND NATURE OF RADIOACTIVITY 



covered by Varley seems, therefore, rather extreme. Varley^^'" cor- 

 rectl}'^ interpreted the cathode rays as " attenuated particles of matter 

 projected from the negative pole by electricity in all directions"; he 

 also caused these rays to produce motion by deflecting a thin plate of 

 talc suspended in the vacuum tube by a silk liber. The rays were 

 deflected so as to impinge on the talc by a magnet, and the spot where 

 they struck the talc was observed to become luminous. 



In the year 1879, ^^^ William Crookes"^ published the results of his 

 wonderful experiments on the passage of electricity through very rare 

 gases. These experiments confirmed beyond question * the obser- 

 vation of Varley that, when the gas in a glass tube is exhausted to 

 about one ten-thousandth of an atmosphere, a stream of material 

 particles passes from the negative pole in the tube. It was already 

 well known that a moving conductor carrying a charge of electricity 

 could be deflected by a magnetic force. The stream of particles ob- 

 served by Crookes could be thus deflected, and, furthermore, they 

 could impart motion to a movable object placed in their path, f and could 

 cast a shadow, as Hittorf had previously shown. Crookes concluded 

 that they revealed a fourth state of matter. " The phenomena in 

 these exhausted tubes reveal to physical science a new world — a 

 world where matter may exist in a fourth state, where the corpuscu- 

 lar theory of light may be true, where light does not always move 

 in straight lines, but where we can never enter and with which we 

 must be content to observe and experiment from the outside."^" It 

 was to this stream of negatively charged particles that the name 

 cathode rays was given. " We have actually touched the border- 

 land," said Crookes, " where Matter and Force seem to merge into 

 one another. . . ."^^ 



Perrin,^®' ^'' some years after, showed that the particles composing 

 these rays carry charges of negative electricity. Two years later 

 Thomson ^^^ verified the work of Perrin, and determined a most im- 

 portant figure, viz., the value of the ratio of the charge to the mass 

 (e/m) of an individual particle from the cathode, and ascertained the 



*The German school was slow to abandon its position that the phenomena in a 

 Crookes tube were a kind of ether-wave. The -work of Varley and Crookes proved that 

 hypothesis erroneous, and it is nowhere held at present. 



t Thomson •*" later stated that the rotation of mill-wheels by the bombardment of 

 the cathode rays is to be considered, not as due to momentum imparted by their par- 

 ticles, but to a secondary effect, due to the rays making the vanes hotter on one side 

 than on the other, thus producing a radiometer action. 



