X-RAYS: A SKETCH OF THEIR HISTORY. 
metal foil on which it impinges to a white heat, or to cause motion in lightly- 
suspended objects. If a magnet be brought near the bulb it is deflected in 
the same manner as would be a, flexible wire carrying an electric current. - 
The name “ kathode rays’? was applied to this peculiar radiation by 
Goldstein, and for long the nature of the kathode rays was hotly disputed. 
The old controversy concerning the nature of light, whether it be a corpuscular 
or a waye-motion, was fought over again, but the decision in this case was 
reversed. The rays were proved by Perrin and by J. J. Thomson to carry 
a charge of negative electricity. The charge and mass of the individual 
particles was determined by observation of the deflections they undergo in 
passing through an electric field of measured strength and through a magnetic 
field. (Figs. 1 and 2 illustrate these effects.) These magnitudes, charge 
by negatively charged metal plate. (From rays due to magnetic field. (Ibid.): 
Sommerfeld’s Atombau und Spektral-linien.) ae 
FIG. 1—Deflection of beam of Kathode-rays ee EIG! 2 Curvature of beam of Kathode- 
and mass, and, what is more easily measured, their ratio, have been found 
to be always the same whatever the nature of the metal forming the electrode 
from which the rays emanate, and whatever the nature of the residual gas, 
_ The definiteness and invariability of these negatively-charged particles give 
them a unique importance in the most important scientific problem of the 
present day, viz., the problem of the electrical constitution of matter. 
A special name, “ electron,” has, therefore, been assigned to this entity. 
The charge of an’ electron has been determined by a variety of means, 
-and found identical with that carried by a hydrogen or other mono- 
valent ion in electrolysis; its mass is to that of a hydrogen atom as 1 to 1,850. 
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