. 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 M focused on it. .4 is a movable screen placed in front of D so as to be bombarded by the a-particles. o-particles directed against them. The number of aluminium nuclei struck in this way is relatively extremely small. Rutherford has calculated that only about two a-particles in every million get near enough to the n
. 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 M focused on it. .4 is a movable screen placed in front of D so as to be bombarded by the a-particles. o-particles directed against them. The number of aluminium nuclei struck in this way is relatively extremely small. Rutherford has calculated that only about two a-particles in every million get near enough to the nucleus to be able to break off a piece of hvdrogen, although each passes through about a hundred thousand atoms of aluminium before it is stopped ; that is to say, there are only two reaUy successful hits in a hundred thousand million shots. Elements which may be Transmuted Of the elements experimented on in this way, six only have been artificially disintegrated. They are boron, nitrogen, fluorine, sodium, aluminium, and phosphorus. These bodies have atomic weights expressible by the formulfe 4w x 3 or 4M x 2 where n is an integer and, since 4 is the atomic weight of helium, their nuclei may be regarded as made up of helium nuclei with 3 or 2 attendant hydrogen nuclei. have been artificially disintegrated and four of these are very common elements. The disintegration or transmutation appears to be quite genuine. In all cases the piece knocked out is the element hydrogen, and the piece that remains becomes a new element. In some cases there is energy " tapped " from the source of internal energy of the atom. But the only means of procuring this type of transmutation arti- ficially is the high-speed a-particles emitted by radio- elements, and these are and alwaj's wiU be among the verj- rarest of substances. So that, although all these processes are of great interest to the theoretical scien- tist, there has been just enough done to encourage the exploiter of scie
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