RECENT DISCOVERIES IN RADIATION. 485 



was mixed with the barium in the precipitate. She therefore an- 

 nounced definitely the discovery of a new substance which she named 

 radium. 



There is a second process, which is now more commonly used than 

 the above, for separating this active substance, that is, the radium, 

 from the barium with which it is always found. It is called the process 

 of fractional crystallization, and consists simply in retaining the first 

 crystals which crystallize out from an active barium chloride solution 

 and then redissolving these crystals and allowing some of them to 

 crystallize out again, and so on. With each new crystallization the 

 activity of the crystals per unit of weight increases. In this way the 

 Curies have recently obtained samples of radium which are as much as 

 1,800,000 times as active as uranium, the activity being measured by 

 comparing the rates at which equal weights of radium and uranium 

 will discharge an electrified body. 



Having followed in this way the processes by which radium was 

 discovered as early as 1898, let us turn to some of the other results 

 which followed close upon the discovery of the X-rays, and which it is 

 necessary to understand something about before we can intelligently 

 discuss the nature of the radiation from radium and other radio-active 

 substances. 



The Nature of Cathode Rays. 



1 have said that X-rays are emitted by an exhausted bulb in which 

 an electrical discharge is passing, but the very existence of X-rays is 

 found to depend upon another kind of rays which are also connected 

 with the electrical discharge from an exhausted tube. These are called 

 the cathode rays because they originate in the negative electrode or 

 cathode, see Fig. 1, of a discharge tube when it is put into connection 



Fig. 1. Illustrating Deflection of Cathode Rays by an Electrostatic Field. 



with an induction coil or static machine. These cathode rays were 

 discovered long before X-rays. Fig. 1 will give some idea of how they 

 manifest themselves. If A and B are two diaphragms, in the middle 

 of which are two horizontal slits, then, when an induction coil is con- 

 nected to the points marked + and — and set into operation, a small 

 spot of greenish-yellow light will appear on the glass at P, just as though 

 some sort of rays were emitted in straight lines from C, and, passing 

 through the two openings 0, fell upon the point P. There are a great 

 many substances which, if placed anywhere in the line OP so that these 

 cathode rays from C can strike upon them, will light up with a charac- 



