What, then, is the nature of these radiations? Are they actually material particles hurled through the ether? Or are they like light--and possibly the Roentgen rays--simply undulations in the ether? As yet this question is an open one, although several of the leading investigators have postulated tentative hypotheses which at least serve as a working basis until they are either confirmed or supplanted. On one point, however, there seems to be unanimity of opinion--there seems to be little question that there are at least three different kinds of rays produced by radio-active substances. According to Sir William Crookes, the first of these are free electrons, or matter in an ultra-gaseous state, as shown in the cathode stream. These particles are extremely minute. They carry a negative charge of electricity, and are identified with the electric corpuscles of Thompson. Rays of the second kind are comparable in size to the hydrogen atom, and are positively electrified. These are easily checked by material obstructions, although they render the air a conductor and affect photographic plates. The third are very penetrating rays, which are not deflected by electricity and which are seemingly identical with Roentgen rays. Professor E. Rutherford has named these rays beta (B), alpha (a), and gamma (v) rays respectively. Of these the beta rays are deviated strongly by the magnetic field, the alpha much less so--very slightly, in fact--while the gamma rays are not affected at all. The action of these three different sets of rays upon certain substances is not the same, the beta and gamma rays acting strongly upon barium platinocyanide, but feebly on Sidot's blende, while the alpha rays act exactly the reverse of this, acting strongly on Sidot's blende. If a surface is coated with Sidot's blende and held near a piece of radium nitrate, the coated surface begins to glow. If now it is examined with a lens, brilliant sparks or points can be seen. As the radium is brought closer and closer these sparks increase in number, until, as Sir William Crookes says, we seem to be witnessing a bombardment of flying atoms hurled from the radium against the surface of the blende. A little instrument called a spinthariscope, devised by Dr. Crookes and on sale at the instrument and optical-goods shops, may be had for a trifling sum. It is fitted with a lens focused upon a bit of Sidot's blende and radium nitrate, and in a dark room shows these beautiful scintillations "like a shower of stars." A still less expensive but similar device is now made in the form of a microscopic slide, to be used with the ordinary lens.