of the constituents of an atom seemed to constitute a difficulty, for it suggested that an atom of matter was not really a permanent and eternal thing, but that it contained within itself the seeds of its own decay and ultimate dissipation into the separate electrons of which it was composed. The process might indeed be exceedingly slow, the radiation loss might be almost imperceptible, but, in so far as an atom is composed of revolving electrons, it is inevitable that radiation of energy must go on from it, and that this must in the long run have some perceptible degenerative result.
10. That result has quite recently, I believe, been experimentally discovered, and is a part of the phenomenon known as 'radio-activity.'
So now we come to the most remarkable and probably the most interesting step of all.
The phenomenon of spontaneous radio-activity, discovered first by Becquerel in uranium and thorium, and greatly extended by the brilliant chemical researches of M. and Mme. Curie which resulted in the discovery of radium (they are coming to London next week, and will be received, I expect, royally by the scientific world), was at first supposed to consist in the emission of a sort of X-rays or ether pulses; and was subsequently assumed to consist chiefly in the bodily emission of electrons, which were shot off from the radio-active substance as they are from a negative electrode in a vacuum-tube, or as they are in air when ultra-violet light falls upon clean negatively charged surfaces.
As a matter of fact both these modes of radiation—the wave form and the corpuscular form—are emitted by radio-active bodies, but they turn out to be of subordinate importance, and must be regarded as secondary or subsidiary results of the main phenomenon.
The main fact of radio-activity has been shown by Professor Rutherford, of Montreal, in a paper published in the month of February this very year, to consist in the flinging away with great violence of actual atoms of matter: atoms electrified indeed, but not negatively like electrons, and not small or penetrating like them, but full-sized atoms, such as are easily stopped by a thin sheet of metal, or even by a sheet of paper—atoms which are positively charged and possessed of a remarkable amount of energy, ionizing the air which they bombard to an extraordinary extent, and likewise generating quite a perceptible amount of heat wherever they strike; producing indeed a flash when they strike a suitable target, as Crookes has shown, quite like the impact of a cannon-ball on an armor-plate. Their speed, indeed, far exceeds that of any cannon-ball that ever existed, being as much faster than a cannon-ball as that is faster than a snail's crawl; a hundred times faster than the fastest flying star, these atomic projectiles constitute the fastest moving matter known. This furious
