In this way it was shown that 3.4 x 10^10 α-particles are expelled per second from one gram of radium itself and from each of its α-ray products in equilibrium with it. The correctness of this method was indicated by another, quite distinct method of counting. Sir William Crookes and Elster and Geitel had shown that the α-particles falling on a screen of phosphorescent zinc sulphide produced a number of scintillations. Using specially prepared screens, Rutherford and Geiger counted the number of these scintillations per second with the aid of a microscope. It was found that, within the limit of experimental error, the number of scintillations per second on a screen agreed with the number of α-particles impinging on it, counted by the electrical method. It was thus clear that each α-particle produced a visible scintillation on the screen, and that either the electrical or the optical method could be used for counting the α-particles. Apart from the purpose for which these experiments were made, the results are of great interest and importance, for it is the first time that it has been found possible to detect a single atom of matter by its electrical and optical effect. This is of course only possible because of the great velocity of the α-particle. Knowing the number of α-particles expelled from radium from the counting experiment, the charge carried by each α-particle was determined by measuring the total positive charge carried by all the α-particles expelled. It was found that each α-particle carried a positive charge of 9.3 x 10^-10 electrostatic units. From a consideration of the experimental evidence of the charge carried by the ions in gases, it was concluded that the α-particle did carry two unit charges, and that the unit charge carried by the hydrogen atom was equal to 4.65 x 10^-10 units. From a comparison of the known value of _e/m_ for the α-particle with that of the hydrogen atom, it follows that an α-particle is a projected atom of helium carrying two charges, or, to express it in another way, the α-particle, after its charge is neutralized, is a helium atom.