The boundary lines of the crystals or grains may be plainly seen. Each grain should show practically white. The dark parallel lines, the dots, and the grayish portions result from inequalities in the polishing and etching. [Illustration: No. 1d. SECTION OF WROUGHT IRON CUT LENGTHWISE OF THE BAR. BLACK PATCHES AND FILAMENTS ARE “SLAG” OR “CINDER” ] After noting the appearance of photomicrograph No. 99b, which is of a nearly pure iron, one need have no difficulty in realizing that pig iron and the steels are alloys and not simple metals. The truth is that of all alloys some of the well-known iron products which we are studying are by far the most complicated, much more so than are the nonferrous alloys, which include the brasses, bronzes, babbitts, German silver, etc. [Illustration: PHYSICAL TEST ROOM WHERE 15,000 SPECIMENS ARE TESTED EACH YEAR ] This should not worry us, however, for we shall not attempt to follow them into their complications. [Illustration: No. 3b. STEEL CONTAINING .10 PER CENT OF CARBON. THE CARBON IS IN THE BLACK PATCHES ] The important point just now is to observe the crooked black flakes of crystalline graphite in this photomicrograph No. 198. It is largely because of these flakes of brittle, soft graphite that pig iron and the cast irons are so fragile. One has no difficulty in realizing that these flakes, which cut in every direction through the metal, make it structurally weak, especially toward a sudden blow. No. 1d shows a typical section of _wrought iron_ cut lengthwise of the rolled bar. It will be noted that, as in photomicrograph No. 99b, there are no graphite flakes. In the process of wrought iron manufacture practically all of the carbon of the original pig iron is burned out, leaving little besides the iron itself and some viscous cinder or slag. In the rolling or hammering out of the resulting white-hot “bloom,” the slag enclosures which remain after the squeezing process are extended lengthwise through the bar. Upon observing with the microscope any prepared section of wrought iron which has been cut lengthwise of the bar the filaments of slag may be plainly seen, all parallel or practically so. When such filaments of slag can be discerned in a longitudinal section it is practically an absolute indication that the material in question is wrought iron. [Illustration: No. 22c. STEEL WITH .50 PER CENT OF CARBON ] [Illustration: No. 36a. STEEL WITH 1.98 PER CENT OF CARBON ] Photomicrograph No. 3b is that of _mild steel_ containing .10 per cent (⅒ per cent) of carbon. Here we have neither the graphite flakes of No. 198 nor the slag filaments of No. 1d. We can plainly see the boundary lines of the grains. The irregular dark patches which are evenly distributed throughout are the defining features of steel. In what might be called a chemical-mechanical combination, these dark patches contain all of the small percentage of carbon which gives to carbon steel its definite properties.