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: [Illustration: FIG. 7.--APPARATUS USED IN EXPERIMENT BY RUTHERFORD AND ROYDS.] A large quantity of radium emanation was compressed into a fine glass tube _A_, about 1.5 cm. long. This tube, which was sealed to a larger capillary tube _B_, was sufficiently thin to allow the alpha particles from the emanation and its products to pass through, but sufficiently thick to withstand atmospheric pressure. The thickness of the glass wall was in most cases less than .01 mm. On introducing the emanation into the tube, the escape of the alpha particles from the emanation was clearly seen by the scintillations produced at some distance on a zinc sulphide screen. After this test the glass tube _A_ was surrounded by a glass tube _T_ and a small spectrum tube _V_ attached to it. The tube _T_ was exhausted to a charcoal vacuum. By means of the mercury column _H_, the gases in the tube _T_ could at any time be compressed into the spectrum tube _V_ and the nature of the gases which had been produced determined spectroscopically. It was found that two days after the introduction of the emanation into _A_ the spectrum showed the yellow line of helium, and after six days the whole helium spectrum was observed. In order to be certain that the helium, coming possibly from some other source, had not diffused through the thin walls of the tube _A_, the emanation was pumped out and helium substituted. No trace of helium could be observed in the vacuum tube after several days, showing that the helium observed in the first experiment must have originated from the alpha particles