Page:Popular Science Monthly Volume 47.djvu/320

 influence of light is at the basis of many photographic processes, notably those of making blue prints and tintypes. Now, of course, in the meteorites containing native iron, not all of the iron is oxidized, and the iron is contained in its less oxidized condition in the other associated minerals, such as the yellowish-green mineral known as chrysolite, sometimes used for a gem. In

 —illustrating (a) the tidal effect on a rigid earth with a fluid envelope, (b) on a yielding earth.

general, also (there are exceptions), the rocks which contain less silica and more iron have their iron less oxidized. By analogy, as we go from the oxidizing effect of the atmosphere toward the center of the earth, we may expect finally to encounter rocks not oxidized even in the less degree. To sum the argument up in a nutshell, we find among the rocks furnished us by volcanic and igneous agencies from various depths in the earth a series from quartzose and feldspathic rocks to those with less quartz and feldspar, more iron, less oxygen, and greater weight, in which the presence of a trace of nickel and the occasional occurrence of diamonds and native iron betoken a kinship to the meteorites. The latter in every way continue this series toward a goal which is nearly pure iron, and the weight of the earth as a whole is consistent with this idea that it is largely iron, almost purely so at the center, but gradually, perhaps not perfectly uniformly, growing more quartzose toward the crust.

One question still remains to us: In what condition is the interior of the earth? Is it a molten fluid or what? If we look at the downward increase in temperature alone it would seem as if within thirty miles a heat would be reached where even pure iron, which is much less fusible than cast iron containing carbon, would be quite fluid. If the earth were freely fluid, however, it