By breaking off pieces of a quenched piece and very carefully and slowly grinding and polishing without heating a surface which was an interior part we find after etching that we can actually see the kind of structure which corresponded with the temperature from which the piece was quenched. Photomicrograph No. 80 shows the appearance of a piece of hardened carbon steel. Note the needle-like structure under the microscope at magnification of 400 diameters. This structure is characteristic. The constituent, having this needle-like appearance has been named “martensite” in memory of a distinguished European metallurgist, A. Martens. It is supposed to be “beta” iron, much the hardest allotropic variety of iron, and to hold in solution the carbon of the alloy, either as carbon alone or as the extremely hard chemical compound, iron carbide, Fe_{3}C. Martensite, then, is the extremely hard structure, necessarily containing considerable carbon or iron carbide in solution which gives to our carbon tool steels their hardness and great usefulness. Unhardened steels never look like this. Their appearance is shown in photomicrographs Nos. 3b, 5, 22 and 24a. [Illustration: No. 80. MARTENSITE, THE CONSTITUENT OF HARDENED STEEL (_Magnification 400 Diameters_) ] In unhardened steels having less than .90% of carbon we find two constituents. “Ferrite” is the name which has been given to one, the soft and ductile constituent, pure iron. With ordinary etching the ferrite usually shows as light-colored or white grains bounded by black lines, which, if the patch is large enough, give a fish-net appearance. It is soft and ductile like copper, for pure iron and pure copper are not so greatly different in malleability and ductility as one might suppose. The darker and more or less triangular patches at the corners of the ferrite grains are “pearlite,” a name originating because of their “pearly” appearance under the microscope. How this pearly appearance comes about will be readily understood from photomicrograph No. 23e which was taken at a magnification of 400 diameters. It is seen that it results from alternate black and white layers. Again we must give up the idea of any finality in the things we learn or think we have learned. We just learned, for instance, that ferrite usually was light or white in color. Well, in pearlite, as shown in photomicrograph No. 23e, every other plate is of ferrite but they are not the white but the black ones. [Illustration: No. 23e. PEARLITE AT MAGNIFICATION OF 400 DIAMETERS ]