Identification of the Rays

To get an understanding of the disintegration occurring in radio-active substances, the nature of the rays produced must be known. These rays are the cause of the activity and their emission accompanies the changes or disintegration. They have for the sake of convenience been called the alpha, beta, and gamma rays. The gamma rays have been identified with the X rays discovered by Röntgen and are a form of energy analogous to light. The beta rays are particles of negative electricity or electrons. With these, then, we have some degree of familiarity. But what are the alpha rays? An answer to this question should make clearer the character of the changes taking place, and should give some insight into the composition and mechanism of the atom.

The Alpha Rays

It has already been stated that these alpha rays are similar or analogous to the canal rays, but this advances the matter very little, as the nature of these canal rays has not been fully determined. The full identity with them, if proved, should have an important theoretical bearing.

Alpha Rays Consist of Solid Particles

In the first place, these alpha rays have been found to be made up of solid particles, that is, of what we are accustomed to call matter. Since it has become more and more difficult to draw a clear distinction between matter and energy, it would perhaps be better to say that these particles appear to have some of the properties hitherto attributed solely to matter. The best evidence that these particles are of atomic mass is furnished by their deflection in electric and magnetic fields.

Electrical Charge

It is not of first importance to discuss this or other proofs of the material nature of these particles. That they carry a charge of positive electricity is, however, a fact of very great import. The value of this charge has been carefully determined by a number of investigators working with different sources of the alpha particles and has been found to be 9.3 × 10-10 electrostatic units (.000,000,000,93 e.s.). From the consideration of the charge upon an electron previously obtained by J. J. Thomson and others, it was concluded that the alpha particle carried two unit positive charges; the fundamental unit charge, therefore, is half this value, or 4.65 × 10-10 e.s.

Helium Formed from Alpha Particles

To determine the nature of the alpha particle a crucial experiment was carried out by Rutherford and Royds, which was described as follows: