a, /3 AND 7 RAYS 



121 



helivim 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 a few centimetres of air. The a rays carry a positive 

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

 The ft 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. negative charges of electricity. 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 vacuum 

 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 power of ionising a gas which is a 

 common characteristic of the radiations 

 from radio - active matter is used as a 

 means for measuring the intensity of 

 radiation. The simplest apparatus for 

 this purpose is a gold-leaf electroscope. 

 Fig. 22 represents the type of electroscope 

 used by Soddy. It consists of a tin can 

 with a movable bottom E for the inser- 

 tion of the substance to be tested. A 

 paraffined rubber cork //, is pierced in inine the ionising power of 

 the centre by the metal wire G which 

 carries at its end a rod of fused quartz, A. 

 A thin brass strip, B, to which a single gold leaf, C, is attached is 

 fastened to the lower end of the quartz rod. F is a vulcanite 

 handle by means of which the charging rod D can be brought into 

 contact with B. The rate of collapse of the gold leaf may be 

 observed by means of a reading microscope through a window in 

 the can (dotted line). 



In 1903, the Curies, who were examining the minerals containing 

 uranium, discovered a new element, radium, in pitchblende. 

 This very radio-active material was obtained pure in 1911. From 

 a ton of pitchblende may be extracted about 200 mgrms. of 

 radium chloride, which was responsible for over 80 per cent, of 

 the radio-activity of the raw material. 



FIG. 22.-Section through . 

 leaf electroscope as used to deter- 





