was mixed with the barium in the precipitate. She therefore announced 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
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 connected 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 O, 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-
