. Discovery. Science. 202 DISCOVERY to describe. The apparatus he used is shown in Fig. i. He found that if a thin piece of magnesium foil, equiva- lent in stopping power to 7 cm. or more of air, were interposed between the source of a-particles and the zinc sulphide screen, on which they could be detected, the screen was quite dark. This is, of course, an old result, and what was to be expected. But the new result was this, that when aluminium foil was substituted for the magnesium many scintillations apf)eared on the screen, and further examination showed that the particles producing these s
. Discovery. Science. 202 DISCOVERY to describe. The apparatus he used is shown in Fig. i. He found that if a thin piece of magnesium foil, equiva- lent in stopping power to 7 cm. or more of air, were interposed between the source of a-particles and the zinc sulphide screen, on which they could be detected, the screen was quite dark. This is, of course, an old result, and what was to be expected. But the new result was this, that when aluminium foil was substituted for the magnesium many scintillations apf)eared on the screen, and further examination showed that the particles producing these scintillations were not a-particles, but particles that could travel as far as 90 cm. in air before they were completeh' stopped. They were proved to be hydrogen nuclei which must have been broken off from those aluminium nuclei which had had a head-on collision with the •It is probably the latter which are ejected when the a-particle strikes. The elements whose atomic weights are less than 32 but not expressible by the above for- mulae, do not appear to be disintegrated by a-particles, and all elements with atomic weights greater than 32 are similarly inert. In the case of aluminium, which shows the phenomenon best of aU, it has been sho\vTi that the energy of the liberated hydrogen is greater than that of the liberating a-particle ; which means that some of the internal energy of the atom has been tapped. This liberation of internal energy occurs, as has been mentioned, spontaneouslv in radio-active bodies, but it is important to have direct proof of it from those elements artificiaUy disintegrated. Let me say this again in different words. Of the eighty-seven known chemical elements, six at present. M la a glass tube which may be evacuated is inserted a disc D containing the body which expels a-particles. It can be brought nearer 5 by the screw R. S is the screen which emits light when stmck by the particles from D. The flashes are observed by looking through the microscope
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