Page:Popular Science Monthly Volume 84.djvu/620

616 can be directly inferred from those of the Brownian particles.

Much more spectacular is the evidence afforded by the cathode rays developed in Crooke's tubes. In these tubes there is emitted from the cathode a stream of luminosity which has very remarkable properties especially that of being deflected by a magnet. This shows that the luminous ray is composed of material particles in motion and charged with electricity. Just to what extent these particles consist of ordinary matter and in how far merely of the electric charge is more or less problematical, but many physicists consider the moving entities as atoms of electricity and this also appears to be Sir Ernest Rutherford's view. These particles are now usually called electrons and they have been identified with the so-called beta rays emitted by radioactive products.

Radium results from the degeneration of uranium, though there are intermediate products, and radium itself likewise gives rise to a series of radioactive products differing from one another. In each of these cases of degeneration, the process is similar. Radium decomposes with the emission of two sorts of rays called the alpha rays and the beta rays. The alpha rays are neither more nor less than atoms of the gas helium, long since known to exist in the sum by its spectrum, and more recently detected in a uranium ore. The beta particles are identical with the electrons which form the cathode rays. The alpha particles are expelled from the radium at a tremendous velocity, but this is far exceeded by the velocity of the beta rays. Sir Ernest Rutherford and his colleagues in radiological investigation have succeeded not only in determining the identity of the alpha particles with helium, but also in establishing the relative size of the electrons and the atoms of helium. The mass of the beta particles is only about one seven-thousandth part the size of the helium atom, and most of the heating effect of radium is due to the energy of the larger alpha particles.

So far has the analysis of these products progressed, and so delicate is the apparatus devised for the study, that Rutherford and Geiger have actually succeeded in making either alpha or beta particles one by one give rise to electrical discharges and light in such a way that the number of either kind of particles emitted per second from a given mass of radioactive matter can be counted. The most efficient apparatus for this purpose is called the string electrometer, so designed as to give a record consisting of small notches on a continuous line. It is like the record of a chronograph and in fact the instrument may be considered as a chronograph. Of this record the notches produced by the alpha particles greatly exceed in depth those given by the beta particles, and thus the rate at which each is given off can be studied with the utmost accuracy on a permanent record.

In a popular description of this kind it is difficult to convey an idea of the extraordinary sensitiveness of the apparatus devised, and none at all of the genius which was requisite to its development, but perhaps enough has been said to show that the most carefully hidden secrets of the ultra-microscopic structure of matter are now subject to scrutiny, and that before long many of its features will be fairly well understood. Sir Ernest concluded his lecture by an illustration of the number of atoms contained in a cubic centimeter of helium. It was something like this. If one hundred million people were to undertake to count these atoms, each person enumerating four per second day and night, the tale would be complete in a couple of thousand years.

second lecture dealt with the problems of the structure of the atom and the bearings of recent researches on this subject. The lecture was most eloquent and left the audience in a condition of the greatest enthusiasm which they testified by