EXPLORING THE ATOM 



tides are streaming. A perforation being made in 

 the electric cathode, these retrograde particles pass 

 through the aperture and impinge on the glass bulb 

 back of the cathode. Because of the method in 

 which they are tested these rays have been called 

 "canal rays" by the German physicist Goldstein who 

 first observed them. . 



Sir Joseph Thomson tested these rays by subject- 

 ing them to the simultaneous influence of an electric 

 current and a magnetic field. Electricity deflects 

 them in one direction and magnetism in another, so 

 that as a result they are diverted from their direct 

 path and assume an elliptical orbit. The record of 

 their divergent flight is instantaneously impressed 

 on a photographic plate. 



To casual observation the photograph suggests a 

 shower of shooting stars or tiny comets, or in some 

 cases an auroral display. But Sir Joseph Thomson 

 has been able to demonstrate that each streak of 

 light represents the flight of a particular type of 

 atom, and that different atoms are deflected in de- 

 grees precisely dependent upon their atomic weight. 

 The hydrogen atom, for example, being very light, 

 is deflected more than the helium atom, and this in 

 turn is deflected more than the still heavier atom of 

 oxygen. 



So here again the individual atoms are made to 

 record their presence. 



The method has great interest for the chemist be- 

 cause it enables him to detect the presence of quan- 

 tities of a foreign gas too minute to be indicated by 

 the spectroscope. The rays are registered within 



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