a, ft AND 7 RAYS 



121 



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 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 ft 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 7 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 F _ 



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 FIO. 22.- -section through goid- 



rr j 11 i TT ' A ' leaf electroscope as used to deter- 



paraffmed rubber cork //, is pierced in mine the ionising power of radio- 

 the centre by the metal wire G which 2dg .^J&eSS^cSffl 

 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. 



