42 



SCIENCE. 



[N. S. Vol. XVIir. No. 445. 



substances are permanently active in a 

 strict sense is not to be regarded as settled. 

 I tbink, however, that most ph.ysicists feel 

 that all active substances must gradually 

 lose their activity, even though the rate 

 of loss is too small to have been yet de- 

 tected. 



A third group of substances contains 

 those which possess temporary activity, 

 lasting for a period ranging from a few 

 minutes to several months. Temporary ac- 

 tivity may be acquired in a large number 

 of ways. It may be obtained from the at- 

 mosphere, from freshly fallen rain or snow, 

 from certain products developed by chem- 

 ical pi'oeesses from other active s^ibstances, 

 and in a variety of other ways. 



In dealing with the effects of the rays 

 produced by radioactive substances we may 

 conveniently adopt the classification shown 

 in the list given below. 



Effects of Becqueeel Rays. 



Photographic action. 



Electric effects. (The most important of these 

 is the power possessed by the rays of making air 

 and otlier gases temporarily conducting.) 



Luminous effects. (Fluorescence produced by 

 the rays in various substances.) 



Chemical effects. (Development of ozone, color 

 changes produced iii glass, etc.) 



Physiological effects. (Burns produced by long 

 exposure to the rays; sensation of light produced 

 by highly active preparations held near the eye.) 



Like Roentgen rays, the rays sent out by 

 radioactive substances do not show regular 

 reflection or refraction. 



It will be noticed that practically all of 

 the effects produced by Becquerel rays are 

 also produced by X-rays. It would be 

 natural to conclude that the rays are of 

 the same type. Yet there are enough differ- 

 ences in the properties of the two rays to 

 show that this can not be true. For ex- 

 ample, Becquerel rays are deflected by a 

 magnetic field and by an electric field and 

 carry an electric charge. X-rays possess 



none of these properties. It is probable 

 that a radioactive substance sends out some 

 X-rays ; but the bulk of the rays emitted 

 by it are of a dift'erent kind. 



The methods used in studying Becquerel 

 rays are naturally based upon the various 

 effects which these rays produce. Up to 

 the present time the photographic effect 

 and the electrical effect have been the ones 

 chiefly employed in the study of the rays. 

 Of the two the electrical method is capable 

 of far greater sensitiveness. In brief, this 

 method is applied in the following way: 

 An insulated conductor of small capacity 

 is connected with a sensitive electrometer 

 and is then charged to a potential of one 

 to two hundred volts. This conductor is 

 placed in a metallic vessel whose walls are 

 grounded. With good insulation the con- 

 ductor will hold its charge under ordinary 

 circumstances for a long period; but if 

 Becquerel rays are allowed to enter the 

 vessel they make the air a conductor and 

 thus permit the charge to escape. The 

 rate at which the charge escapes, as indi- 

 cated by the electrometer, is a measure of 

 the intensity of the rays. 



In the early study of the subject dif- 

 ferent observers often obtained contra- 

 dictory results. In many cases the con- 

 tradictions have since been explained by 

 the fact that some used the photographic 

 method while others used the electrical 

 method. The two methods of measuring 

 the intensity of the rays do not agree. 

 For example, a substance A may produce 

 very strong photographic effects, while an- 

 other substance B, also tested photograph- 

 icallj', is found to give out rays that are 

 relatively weak. But if the two sub- 

 stances are compared by means of the 

 electrical effects which they produce, it may 

 turn out that B is more active than A. 



Results such as this have led to the con- 

 clusion that there are different kinds of 



