1 58 RAD 10- A CTI VI T Y 



stances are practically insulators, capable of conducting limited 

 quantities of either positive or negative electricity. 



Poincare suggested that the production of X-rays might be an 

 effect common to all fluorescence. In 1896, Becquerel, acting 

 on this idea, examined some fluorescent salts of uranium. He 

 found that the double sulphate of uranium and potassium exposed 

 to sunlight could affect a sensitised plate even when the plate was 

 protected by a layer of copper or aluminium foil. This metallic 

 layer excluded the possibility of action by ultra-violet light or by 

 chemical vapours emitted by the salt. Further investigation 

 showed that the phenomenon was exhibited by uranow^ salts 

 which are not fluorescent, as well as by the fluorescing uranic 

 salts. Both are active in proportion to the amount of uranium 

 they contain. That is, the continuous emission of these rays is 

 a specific property of uranium now generally termed Radio- 

 activity. 



The characteristics of the radiation from uranium are very 

 similar to those of the X-rays. They are found to consist of three 

 very distinct types of rays, differentiated in the first instance by 

 their power of penetrating matter. They have been termed by 

 Rutherford, a, /S and y rays. The a rays arc particles of the gas 

 helium expelled radially from the uranium with the colossal 

 speed of 20,000 miles a second. They have so feeble a penetrating 

 power that they are completely stopped by a single sheet of note- 

 paper or by about 7 cm. of air. The a rays carry a positive 

 charge, but are only slightly deviable by an intense magnetic field. 

 The ^ rays resemble the X-rays in penetrating power, and pass 

 with ease through thin metal, glass, etc., but are nearly all stopped 

 by a single coin. Becquerel proved that the ^ rays are identical 

 with the kathode rays, i.e. electrons. Their superior penetrating 

 power is due to their enormously greater velocity. The y rays are 

 not deflected by magnetic fields. They resemble in all respects the 

 X-rays, but are far more penetrating than rays even from the 

 hardest vaciumi tube. They will readily pass through a pile of 

 twelve coins. Their nature is probably the same as that of 

 X-rays, i.e. thin pulses in the ether. 



The a and ^ rays do not penetrate gases by pushing aside those 

 molecules of the gas that lie in their path. They actually pass 

 through the molecules (which, of course, are mostly " hole ") and 

 knock off, in their progress, some of the outlying electrons. In 

 passing through 7 cm. of air, the a particle chips off about 130.000 

 electrons and so " ionises " the air. 



This power of ionising a gas is used as a means for measuring the 

 intensity of radiation. The simplest apparatus for this purpose is 



