544 SMITH'S INTERMEDIATE CHEMISTRY 



helium gas given off by radium compounds was collected by 

 Soddy working with Ramsay and identified, and its rate of pro- 

 duction was measured. The amount was equal to 158 cubic 

 mm. per 1 g. of radium per year. 



The beta-particles are electrons (p. 195) , or unit charges of 

 negative electricity, and are shot out with a velocity approach- 

 ing that of light (300,000 kiloms. per sec.). Their apparent mass 

 is very small (about T ^^ that of an atom of hydrogen) . Owing 

 to collisions with the relatively ponderous air-molecules, half of 

 them are lost after going about 4 cm. 



The gamma-rays are identical with X-rays (see p. 548), and are 

 presumably produced like the latter by the impacts of the elec- 

 trons on the surrounding matter. 



The helium atoms are almost all stopped by a sheet of paper or 

 by aluminium foil 0.1 mm. thick. The electrons have greater 

 penetrating power, many passing through gold-leaf, but being 

 practically all blocked by a sheet of aluminium 1 cm. thick. 

 The gamma-rays (X-rays), however, are able to penetrate rela- 

 tively thick layers of metals and other materials of low atomic 

 weight. 



One of the most striking facts is that the stoppage by the air 

 of so many rapidly moving particles results in the production of 

 much heat. One gram of radium would produce about 120 caL 

 per hour. 



Disintegration. The emission of atoms of helium and of 

 electrons was first explained by Rutherford (1902-3), then of 

 McGill University, Montreal, as being due to the spontaneous 

 disintegration of the atoms of uranium, radium, and other radio- 

 active elements. Thus, Rutherford was the first to show that 

 radium compounds produced a gaseous substance called the 

 radium emanation (niton), which was the residue left after the 

 emission of one atom of helium from an atom of radium. This 

 gas was itself radioactive and underwent further disintegration, 



