RADIUM, ATOMIC ENERGY, AND ATOMIC STRUCTURE 543 



placed it with the metals of the alkaline earths The ratio by 

 weight of chlorine to radium in the chloride is 35.46 : 113, so that, 

 on the assumption that the element is bivalent, its chloride is 

 RaCl 2 and its atomic weight is 226. With this value it occupies 

 a place formerly vacant in the periodic table. 



In 1910 Mme. Curie obtained metallic radium by electro- 

 lyzing a solution of radium chloride, using a mercury cathode, 

 and expelling the mercury by distillation. It was a white metal 

 (m.-p. 700) which, like calcium, quickly tarnished in the air and 

 displaced hydrogen from water. 



The Nature of the "/Joys." Many properties show that 

 the " rays " emitted by compounds of uranium and of radium are 

 of three kinds. They are most sharply distinguished from one 

 another when allowed to pass through a powerful magnetic field. 

 The alpha-rays are positively charged and are bent in one direction 

 while the beta-rays are negative and are bent in the other. The 

 gamma-rays are not affected. 



The alpha-rays are atoms of helium (p. 297) thrown off in 

 straight lines with varying initial velocities, averaging about one- 

 tenth that of light. Each such atom bears a double positive 

 charge (the unit being the charge on a univalent positive ion), 

 and a delicate electrometer readily indicates the impact of a 

 single atom. These alpha-particles, being each four times as 

 heavy as an atom of hydrogen, plough their way through tens of 

 thousands of air-molecules and usually go about 3-8 cm. before 

 being stopped. The emission of atoms of helium can be de- 

 tected by means of Crookes' spintharoscope (Fig. 120). The 

 particle of radium bromide is at B, and some of ^ ______ 



the charged helium atoms strike a surface C A | B I 



covered with zinc sulphide, producing faint B M 



flashes of light. The lens A magnifies the FIQ 12Q 



flashes, which can be seen in a dark room after 



the eye has become thoroughly rested (15-20 minutes). The 



