RADIOACTIVITY. 119 



only because the substance sought for possessed the exceptional 

 qualit}' of raclioactivitj^ 



A radium crystal emits three different types of radiation, which, 

 RS suggested by Eutherford. one of the most successful investigators 

 in the domain of radioactivity^, have been designated as a, (3, and y 

 rays. The existence of these rays can not be directly perceived by 

 any of the senses; we are only indirectly made aware of their pres- 

 ence by the phosphorescence which the}^ excite, bj' their action on a 

 photographic plate, and by the electrical conductivity which they 

 ji rouse in the atmosphere and in the gases. 



When we see a radium crystal shining like a glowworm in a 

 darkened room, it is because the crystal itself has been excited to 

 fluorescence by the rays, exactly as a Rontgen screen or the Sidot- 

 blende can be made fluorescent by the same means. The sensation 

 of light experienced when the crystal is laid on the temple or on 

 the closed eye is due to the fact that every part of the eye itself, and 

 ■especially the lens, becomes luminescent under the influence of the 

 radium rays. 



The most important property discovered in the investigation of 

 radioactivity is tlie power of the rays to render the atmosphere and 

 the gases, which are generally good insulators, electrically con- 

 ductive. If we charge an insulated body like the ball of a gold 

 leaf electroscope with electricity it will be an hour or more before 

 we can detect any diminution of the charge by the slightest diver- 

 gence of the gold leaf: but if we bring a radium preparation into 

 the vicinity of the instrument the leaves suddenly collapse and the 

 electrical charge disappears almost instantly. If instead of the 

 electroscope we take one of our highly sensitive electrometers, we 

 have a mode of experiment a thousand times more sensitive to the 

 slightest traces of radioactivity than any other known method of 

 l^hysical and chemical analysis. This apparatus, with the helpful 

 aid of photograi^hy, has been of vast use in determining the direc- 

 tion of the a, /?, and y rays, and have thus added materially to our 

 knowledge of the subject. 



The a rays act like positively charged bodies of the size of the 

 chemical atom which are projected from radium with the greatest 

 velocity. They are so slightly penetrating that a sheet of paper is 

 impervious to them, and they are absorbed by a layer of air a few 

 centimeters thick. 



The /3 rays are negatively charged bodies, A^hich in every regard 

 resemble the cathode rays produced in a vacuum tube when an 

 electric charge is sent through it. These rays have become widely 

 known from the fact that the Rontgen rays originate at the point 

 where the cathode rays meet with an obstacle. 



