The second list is the _radio-active_ group of elements,--uranium, radium, thorium, actinium, etc. These all possess their characteristic properties in varying degrees,--giving off alpha, beta and gamma rays. A certain mysterious “emanation” is also emitted by radio-active elements, but the study of these rays and their influence would take us into the realm of “physics,” and would more properly belong to a book on physics than in the present, dealing with chemistry. One very interesting fact should, however, be mentioned in this connection, and that is the evolution of matter which has been observed, as the result of spectrum analysis. We have heard much of organic evolution, meaning the evolution of life upon our planet. It is equally true that there is an inorganic evolution, in which the gradual development of chemical elements may similarly be traced. Thus, it has been noticed that, in the hottest stars, (gaseous) the fewest chemical elements exist; in those of medium temperature (metallic), more elements are found, while in those having the lowest temperature (carbon stars) the greatest number of chemical elements are to be distinguished. This seems to prove that the higher the temperature, the fewer the elements, which in turn leads to the conclusion that all elements are perhaps ultimately ONE--as Sir William Crookes suggested many years ago. As these stars cool, more and more elements seem to be “crystallized out,” so to say,--the many being formed from the fewer. The newer researches on the constitution of matter render this idea all the more plausible. THE ORIGIN OF LIFE Just here, it might be well to point out the late place occupied by life, in this process of inorganic evolution. The Absolute Zero of inter-stellar space is about -273°C. On the other hand, the temperature of the hottest stars is more than 30,000°! (Argo, Alnitam, etc.) The temperature must fall from this to a few degrees above Zero (the boiling point of water), before life can become manifest at all. Life as we know it can only exist between the boiling and the freezing points of water. This point is only reached towards the very end of the scale. It has therefore been said that, cosmically speaking, life is only a “flash in the pan between two eternities”--but for us that flash in the pan is everything! The question of the origin of life upon our planet has been discussed at some length in my little book on “Life: Its Origin and Nature,” in the present series. It may be of interest to mention here, however, a few of the experiments which have been made upon the artificial creation of life, by means of inorganic chemicals, since these properly fall into place in a book devoted to chemistry.