It became obviously a question of the first importance to discover what proportion of the earth’s crust consists of radium, and an investigation was undertaken for this purpose by the Hon. R. J. Strutt, who finds that the rocks composing the earth’s crust contain a superabundance of radium--sufficient, if this element is uniformly distributed through the whole earth in the same proportion as it occurs at the surface, not only to make good the heat which is radiated away into space, but actually to raise the temperature of our planet, which, on this evidence, should, therefore, be growing not colder, but hotter. This is a result as disconcerting at first sight as it is astonishing, and its effects are very wide-reaching. Of course, it completely destroys the validity of Lord Kelvin’s argument, but it also deprives the nebular hypothesis of one of its cherished lines of evidence--a loss which the force of the general argument enables us to bear with equanimity. [Sidenote: On the Eve of great Events] In any case, the vast body of facts bearing on the history of the earth suffices to show that its temperature cannot be rising. Mr. Strutt has, therefore, imagined that the radium is not uniformly distributed throughout the mass of the planet, and supposes that it is restricted to an external zone forty-five miles in thickness; this would suffice to maintain the earth at its existing temperature. If, however, we admit a restriction of this kind, we are in no way bound to fix the limit at forty-five miles. All we can say is that we do not know how far downwards the radium reaches--for aught we know five miles, or even less, is as likely a limit as forty-five miles. Professor Joly, indeed, maintains that the radium we meet with is not proper to the earth at all, but comes from the sun. Radium is a short-lived element, its existence being limited to a few thousand years; but as fast as it decays it is reproduced at the expense of another element--uranium--the lifetime of which is measured by hundreds of millions of years. The last quarter of a century has proved fertile in great discoveries--more so than any corresponding period in the past. As a result, the whole world of scientific thought has been thrown into commotion; old-established theories, and even the most fundamental notions, seem to be in a state of flux. Under the stimulus of new ideas great questions, such as the constitution of matter, the origin of species, and the birth of worlds are being re-investigated with renewed energy, and we seem to be on the eve of great events. WILLIAM JOHNSON SOLLAS FOUR PERIODS OF THE EARTH’S DEVELOPMENT A Postscript to Professor Sollas’s Chapter on the Wonderful Story of the World’s Birth, beginning on page 79 The earth was once “a fluid haze of light.” The whole solar system once formed a vast nebula, consisting of glowing gas, or a swarm of meteoroids. Our planet was slowly shaped into a globe out of this primitive nebula.