Page:Popular Science Monthly Volume 25.djvu/95

Rh and sawing of stones do not necessarily require metallic tools, but were all formerly done with instruments of stone and wood.

It was believed a little while ago that the stone age was superseded by a bronze age. A closer examination of the subject has made it clear that we have in this case to deal not with sharply distinguished intervals, but with different degrees and conditions of civilization, which existed at the same time among different nations, and even among different classes of the same nation. Stone tools and weapons were still in use in Northern Europe long after the hard metals had become common in the South; but even in Southern Europe the poorer classes continued to use stone implements till late in historical times.

Survivals of stone-age civilization are now met with among only a few peoples; men have as a rule advanced to the metal-using stage, which has acquired its significance in consequence of the production of the hard metals. It is our purpose to review the origin, the acquisition, and the application of these important materials.

I have already spoken of the core of the earth as consisting chiefly of iron, while the crust is composed of eruptive silicates. The fluid from which these masses have been derived is a mixture of several combinations, the principal elements of which are oxygen, silicon, the lighter earth and alkali metals, and the heavier metal, iron. Oxygen predominates, and is combined with the other elements into stony oxides. The excess of oxygen floats around the oxidized dead-burned globe as life-air. I have mentioned only iron among the heavy metals important in civilization because it is the one that plays the most important part in the composition of the rocks. We also find smaller masses of other heavy metals in the primitive matter of the earth. Most of them appear sparsely distributed in the eruptive rocks in combination with sulphur, but a few occur in oxides. Besides these we also find ores concentrated in crevices and in pockets in rock-masses of different kinds, where they have been carried in aqueous solution. These local accumulations of ores first made it possible for men to obtain the rarer metals in masses and apply them to use. The metallic sulphurets nearer to the surface have been changed by atmospheric action partly into the simple metals, partly into oxides. This was a fact of great importance in the beginnings of metallurgic art; for the oxides are much easier to reduce to the condition of pure metal than the sulphurets. The unmixed metals—the precious metals and copper—are, of course, immediately available, and can be brought into any form that may be desired by hammering or casting. But little advantage was, however, derived from this circumstance. The first decisive step was not made till the hard metals, bronze and iron, were produced. These substances could not, however, be obtained at once, for neither the tin required for the manufacture of bronze nor iron in the metallic state was at hand; and it was a great step when the thought first occurred to man of separating the metallic elements from their