568 THE POPULAR SCIENCE MONTHLY
possibilities within range of our understanding be exhausted; we can hardly expect to understand a fourth state until we have fathomed the relations between the three states which we already know, and also their intermediate forms which immediately precede their critical points of transformation. We may some day be forced to an acknowledgment of this fourth state, although we may never be able to conceive it. It would be somewhat surprising that the ether be in any form known as matter ; much more surprising than that matter be, after all, but another manifestation of energy.
It is a common thing for writers to dwell upon extinct theories. History is very well in its place, but in this essay extinct and not-gener- ally-accepted theories will be disregarded in favor of those of more recent growth, or such as may be suggested by the recent discoveries in physical science. On this account it will be necessary, in the first place, to review very briefly the present state of radiology, without a knowledge of which a proper understanding of modern theories would be difficult.
The Facts of Eadiologt
Credit must be given to the early work of Sir William Crookes on radiant matter for having prepared the way to recent discoveries in this branch of physical chemistry. Credit must also be given to Sir J. J. Thompson, for work on the electric properties of gases, without which many of the important facts of radiology would have remained either undiscovered or barren, to which have been added his many masterly discoveries in the electronic world.
The most studied radio-elements and the most interesting for the present discussion are uranium, actinium, radium and thorium. The distinctive property of the radio-elements is to disintegrate, forming other radio-elements and also a more stable element. The disintegra- tion is not molecular ; it is atomic ; the atom breaks down, it is destroyed or converted into the atom of another element. Radium is apparently de-energized uranium; the spectrum of uranium is entirely distinct from that of radium and they may therefore be considered distinct chemical elements. The radiations and emanations of all radio-ele- ments being, in a general way, the same, it is only necessary to describe in detail the disintegration of radium, which is the most interesting and complete of all.
The magnet will separate the radiation from radium into three dis- tinct streams, just as a prism will break up white light into its physi- ological primaries. These three radiations are known, respectively, as a, /3 and y radiations, and radiations possessing similar characteristics are given off by all known radio-elements. The a radiation, which ap- pears to be composed of helium atoms, has secondary rays composed