4 SODDY, Evolution of Matter by Radio-active Elements. 



classes to one another is extremely close, and it is only 

 in the action of the magnetic field that the difference is- 

 apparent. Thus, as a first approximation, the different 

 types of radiation penetrate matter in proportion to its 

 density, and independently of its nature. If layers of 

 equal area and equal weight of the various different kinds 

 of matter are compared, the stopping power will be found 

 similar for each. The phenomenon exhibited by light of 

 being able to pass without much loss through great 

 thicknesses of certain kinds of matter known as transparent,, 

 and of being stopped completely by the thinnest sheets 

 of other kinds, is unknown for the new types of radiation. 

 The penetrating power of the latter varies very widely for 

 the different types, the 7 rays being able to pass with 

 ease through an inch of steel, while the a rays are com- 

 pletely stopped by a sheet of notepaper. But the law 

 that the absorption of the radiation is proportional to the- 

 density of the matter traversed, holds approximately for 

 each of the new types. 



The methods used in the detection and investigation- 

 are the same for all the radiations mentioned. All are 

 invisible to the unaided eye, but possess the power of 

 causing certain fluorescent substances placed in their path 

 to emit visible light, or to fluoresce, although this power 

 varies widely with the different substances under the 

 influence of the different kinds of radiation. All the new 

 radiations affect the photographic plate in the same way 

 as light, although again in very different degrees. Their 

 most important property is their power of ionising the air 

 or other gases through which they pass. Under these con- 

 ditions the gases are for the time being made capable of 

 conducting electricity in amounts proportional to the 

 . intensity of the radiation. 



The three properties last mentioned furnish the 



