when an electrical charge is placed upon a body, the medium about the body becomes the seat of new forces, and this may be described by saying that the medium about the body has been thrown into a state of strain. But it is one thing to say that the electrical charge on the body produces a state of strain in the surrounding medium, and quite another thing to say that the electrical charge is nothing but a state of strain in the surrounding medium, just as it is one thing to say that when a man stands on a bridge he produces a mechanical strain in the timbers of the bridge, and another thing to say that the man is nothing more than a mechanical strain in the bridge. The practical difference between the two points of view is that in the one case you look for other attributes of the man besides the ability to produce a strain in the bridge, and in the other case you do not look for other attributes. So the strain theory, although not irreconcilable with the atomic hypothesis, was actually antagonistic to it, because it led men to think of the strain as distributed continuously about the surface of the charged body, rather than as radiating from definite spots or centers peppered over the surface of the body. Between 1850 and 1900, then, the physicist was in the following anomalous and inconsistent position: When he was thinking of the passage of electricity through a solution, he pictured to himself definite specks or atoms of electricity as traveling through the solution, each atom of matter carrying an exact multiple of a definite elementary electrical atom; while, when he was thinking of the passage of a current through a metallic conductor, he gave up altogether the atomic hypothesis, and attempted to picture the phenomenon to himself as a continuous "slip" or "breakdown of a strain" in the material of the wire.
About 1900, however, a great stride forward was taken when the atomic hypothesis began to be applied to metallic conductors as well as to solutions, and electrical currents, even in wires, began to be looked upon as due to the transport through the wire of discrete units of electricity, now beginning to be called electrons, these units being either handed on from atom to atom or else being pushed along through the interstices between the atoms. This point of view, which was a return to Franklin's way of thinking, found its new justification in the fact that it was found possible in vacuum tubes of the X-ray type to obtain from all kinds of matter very minute electrically charged bodies of negative sign, which under all circumstances showed exactly the same behavior in electrical and magnetic fields and which had a mass which was computed to be but 1/1,760 the mass of the atom of hydrogen, the smallest known atom of matter. There was indeed no direct proof that the charges of these bodies were all the same, since no method had been found of examining them individually, nevertheless, it was pretty conclusively shown, as early as 1899, by Townsend of Ox-
