Part of the gas, helium, may escape, but except in rare instances where chemical alteration might occur, there probably is no loss of lead. Fortunately, when this metal is produced by radioactivity it differs slightly in atomic weight from ordinary lead; otherwise the presence of the latter would introduce a misleading factor. Since the speed at which the change goes on cannot be increased or decreased, it is assumed that throughout past ages it has never been faster or slower. The amount of such change that has been completed in any body of radioactive minerals may be measured by techniques employed in physics and chemistry. If it is found that the amount of helium or lead present requires a hundred million years for its production at the working speed of the parent elements, the mineral deposit must be at least that old. Certain conditions of course complicate the problem seriously: knowing the age of a piece of rock which happens to contain some radioactive element is of small service in historical studies unless the rock can be definitely associated with a flora or fauna, or some outstanding event disclosed by geological investigations. But there have been a few instances in which most of the necessary conditions were present, and more and better opportunities to apply this method will no doubt appear. Other elements, or their radioactive isotopes, are already being employed with good results. Some of these, such as carbon 14, are more sensitive indicators for the accurate dating of events in comparatively recent time. When it can be used, this type of measurement is far less subject to uncertainties than any other. It promises to eliminate all need for guessing, and comes close to a degree of accuracy which is satisfactory to the scientist, a person who thoroughly dislikes uncertainties of any kind. If suitable material can be found in just the right places it should accomplish what the preceding method cannot do—the accurate measurement of the great time breaks which interrupt the geological record in many places. Something along this line already has been accomplished, for radioactive material has been found in some of the oldest of the rocks. Regardless of the destruction going on in other localities, these rocks have continued to register the passing of time, and a tremendous antiquity for the earth and some of its first inhabitants has been indicated.