it has been shown nmon constituent, "of a fundamental

l mass, about onest atom, bearing a particular atom the of small dimensions qual in amount to the ectrons, so that, under ally neutral. Further, lving about the nucleus re, may be compared not system. The positive he electrons, like planets,

ption, therefore, all atoms he Maxwellian idea of their ly one which has had to go. ve shown that we must also tion of a molecule (or atom) order of nature under which

has shown that the atomic ay become unstable and a porWhat is left behind is then no ..m of a new substance. In other hemists of transmuting matter , modern transmutation is not on nner the alchemists sought after, a process of disintegration, a porhange into another. Occasionally ..ble atoms a catastrophe occurs-a

of a new substance is left behind. of Prof. Rutherford, of Manchester eGill University, is outstanding. Xps.mental evidence supporting the view ator outlined above is almost overwhelmconsidered in an article of this kind. interesting investigations bearing on : have been made at the Cavendish LabA brief account of each of them is given

mon with an atom of iron. On the contrary, it has been shown that atoms of all substances contain a common constituent, that each atom contains a definite number of a fundamental unit, the electron—a particle of very small mass, about onetwo thousandth part of that of the lightest atom, bearing a charge of negative electricity. In any particular atom the electrons are grouped about a nucleus of small dimensions bearing a charge of positive electricity equal in amount to the total negative charge carried by the electrons, so that, under normal conditions, the atom is electrically neutral. Further, the electrons are in rapid motion, revolving about the nucleus as centre. The modern atom, therefore, may be compared not inappropriately to a miniature solar system. The positive nucleus acts as a sun about which the electrons, like planets, execute their orbital motion.

According to this modern conception, therefore, all atoms have very much in common. But the Maxwellian idea of their physical independence is not the only one which has had to go. Investigations in radioactivity have shown that we must also abandon the idea that "the formation of a molecule (or atom) is an event not belonging to the order of nature under which we live.” Research in this field has shown that the atomic orbital system sketched above may become unstable and a portion of an atom be shot off. What is left behind is then no longer the same atom but an atom of a new substance. In other words the dream of the old alchemists of transmuting matter has been realized. It is true, modern transmutation is not on the gross scale nor in the manner the alchemists sought after, but it is equally true that by a process of disintegration, a portion of one substance may change into another. Occasionally in some of the heavier unstable atoms a catastrophe occurs—a portion is shot off, an atom of a new substance is left behind. In this discovery the name of Prof. Rutherford, of Manchester University, formerly of McGill University, is outstanding.

The amount of experimental evidence supporting the view of the structure of matter outlined above is almost overwhelming and can scarcely be considered in an article of this kind. Recently, however, two interesting investigations bearing on this and allied subjects have been made at the Cavendish Laboratory, Cambridge. A brief account of each of them is given below.

RAYS OF POSITIVE ELECTRICITY.

If from a cylindrical glass vessel (figure 1) provided with metallic electrodes A and B at each end, most of the air (or other gas) is exhausted, it is found, on joining the electrodes to the terminals of a sufficiently powerful electric machine, that an electric current passes through the gas. A simple explanation of this phenomenon may be given. On account of the electrical pressure (potential difference) existing between the electrodes, many of the atoms lose one or more of their electrons and are consequently left with an excess of positive electricity. We have then in the vessel, in addition to ordinary neutral atoms, free negative electrons and positively charged atoms, both of which frequently attach themselves to neutral atoms or molecules, thus forming what are called negative and positive ions. The gas in this condition is said to be ionized. Now, since positive electricity attracts negative and repels its own kind, we have a stream of negative ions moving towards the positive electrode and a stream of positive ions in the opposite direction. It is this stream that constitutes the current of electricity in the vessel.

If now the negative electrode perforated with several holes is placed so as to divide the vessel into two parts, as in figure 2, some of the positively charged particles moving towards this electrode will pass through the holes into what we may call the observation chamber. These particles entering the observation chamber are called positive rays. They were first observed in 1886 by Goldstein, a German physicist, who