# Page:Popular Science Monthly Volume 78.djvu/578

578
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-generally-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 magnet will separate the radiation from radium into three distinct streams, just as a prism will break up white light into its physiological primaries. These three radiations are known, respectively, as ${\displaystyle \alpha \,\ \beta }$ and ${\displaystyle \gamma }$ radiations, and radiations possessing similar characteristics are given off by all known radio-elements. The ${\displaystyle \alpha }$ radiation, which appears to be composed of helium atoms, has secondary rays composed