ions have been abundantly confirmed. It appears certain that electric charge exists in discrete units, vitreous and resinous, each of magnitude 1·5 x 10-19 coulombs approximately. Each ion, whether in an electrolytic liquid or in a gas, carries one (or an integral number) of these charges. An electrolytic ion also contains one or more atoms of matter, and a positive gaseous ion has a mass of the same order of magnitude as that of an atom of matter. But it is possible in many ways to produce in a gas negative ions which are not attached to atoms of matter; for these the inertia is only about one-thousandth of the inertia of an atom; and there is reason for believing that even this apparent mass is in its origin purely electrical.[1]
The closing years of the nineteenth century saw the foundation of another branch of experimental science which is closely related to the study of conduction in gases. When Röntgen announced his discovery of the X-rays, and described their power of exciting phosphorescence, a number of other workers commenced to investigate this property more completely. In particular, Henri Becquerel resolved to examine the radiations which are emitted by the phosphorescent double sulphate of uranium and potassium after exposure to the sun. The result was communicated to the French Academy on February 24th, 1896.[2] "Let a photographic plate," he said, " be wrapped in two sheets of very thick black paper, such that the plate is not affected by exposure to the sun for a day. Outside the paper place a quantity of the phosphorescent substance, and expose the whole to the sun for several hours. When the plate is developed, it displays a silhouette of the phosphorescent substance. So the latter must emit radiations which are capable of passing through paper opaque to ordinary light, and of reducing salts of silver."
At this time Becquerel supposed the radiation to have been excited by the exposure of the phosphorescent substance to the sun; but a week later he announced[3] that it persisted for an
