49° POPULAR SCIENCE MONTHLY. 



is known about their nature. Since, however, the energy carried by 

 them is very insignificant as compared with that in the alpha and beta 

 rays, we can leave them entirely out of account in most of the compu- 

 tations which we make upon the energy of radiations of radio-active 

 substances. It is now conjectured that the gamma rays are ethereal 

 pulses like the X-rays. 



The Nature of the Alpha Rays. 

 It was at first conjectured that possibly the alpha rays might be 

 X-rays, since, like them, they are not deflected by a magnet, and since, 

 also like them, they are very effective in rendering a gas electrically 

 conducting. But only last year Professor Kutherford contrived a very 

 ingenious experiment by which he showed conclusively that the alpha 

 rays are deflected very slightly by a magnet if the magnet is suflBciently 

 powerful. He also succeeded in showing that they are deflected by a 

 very strong electrical field. But in both of these cases the direction 

 of the deflection is opposite to that obtained under the same conditions 

 with beta rays. These results of Professor Eutherford's are of the 

 utmost importance, and they have been recently confirmed both by 

 Becquerel in Paris, and by a German physicist by the name of Des 

 Coudres. The only possible interpretation which can be put upon 

 them is that the alpha rays also consist of particles of matter shot off 

 from the radio-active substances, but that, while the beta ray particles 

 carry charges of negative electricity, the alpha ray particles carry 

 charges of positive electricity. 



Further, when from the amounts of the deflections produced by the 

 magnet and by the electric charge, the size and velocity of the alpha 

 particles are calculated, the results are again most interesting. For 

 these particles are found to have a mass not one one-thousandth that of 

 the hydrogen atom, like the cathode rays, but approximately twice as 

 great as that of the hydrogen atom, or about the size of the atom of 

 helium. (The atomic weight of helium is 4.) They are there- 

 fore about 2,000 times as heavy as the cathode ray particles. This ex- 

 plains why they do not pass through ordinary matter as readily as do the 

 smaller beta particles. But despite this comparatively great mass, their 

 velocity is found to be as much as 20,000 miles per second, more than 

 a tenth that of the smaller particles. It will be seen, therefore, that 

 the energy of the blows which they strike against the bodies upon 

 which they fall is much greater than that of the beta particles. This 

 explains why they knock the gas to pieces, or dissociate it and thus 

 render it conducting, so much more energetically than do the beta 

 particles. 



The CrooTces Spinthariscope. 



We have attempted to follow, thus far, the evidence upon which we 

 base the conclusion that the radiations from radioactive substances con- 



