HARDENED BY OVERSTRAIN. 
13 
by figs. A and B seems, however, to indicate that the change in the properties of the 
material has to he attributed more to a thermal cause. It is unlikely that the 
structure of the material could be changed back from the annealed condition {fig. B) 
to the hardened condition (fig. A) by purely mechanical means. Mechanical 
hardening is accomjianied by distortion of the grains, not by change in their 
dimensions. 
The second method of microscopic examination which was adopted may lie 
classified as what Osmond calls un polissage-attaque.” The process was dis¬ 
covered accidentally, and consists simply in rubbing a surface of steel polished with 
wet rouge in the manner described above, with ordinary moistened cocoa. The 
cocoa stains the pearlite areas of the steel, which are thrown into relief by the 
jiolishing, the effect being probably very analogous to that produced liy the infusion 
of liquorice root, which Osmond and others usually employ. Fig. C (Plate 1) shows, 
under a magnification of 150 diameters and with vertical illumination, the structure 
of the 1-inch steel rod whose properties are given in the preceding section of this 
paper; the surface examined rvas polished with emery and rouge in the ordinary 
manner, and then rubbed for a little while on a piece of chamois leather which had 
been moistened and covered with Van Houten’s cocoa. The surface was also 
examined under a magnification of about 3000 diameters, and the beautiful laminated 
structure of the pearlite areas was thus very clearly shown. One series of the 
laminre—the Sorbite series—were stained a brown colour, the other series remained 
bright. It was found that benzene removed the staining produced by the cocoa. 
The specimens from which figs. A and B had been obtained were repolished to 
remove the effect of the etching by nitric acid, and were then stained with cocoa in 
the manner just described, and examined under magnifications of 150 and of 3000 
diameters. The structures shown were much better defined than when produced by 
ordinary etching, and under tlie high magnification tlie laminated nature of the 
pearlite areas was clearly visible. The laminm were more distinct in the annealed, 
that is in the coarser grained specimen. 
The third method of microscopic examination which was adopted consisted simply 
in polishing the steel in the ordinary way with emery and wet rouge, and then 
drying the surface and rubl)ing it on a leather pad coated with dry rouge. The 
polishing with wet rouge on chamois leather brought the surface into bas-relief, the 
rubbing on the dry pad filled the hollows with fine rouge, so that when examined 
under a magnification of 150 diameters the structure of the steel could readily be 
seen. The difference between the structures slioum in figs. A and B was quite as 
clearly and accurately shown by this means as by etching or by staining, but of 
course under hio-h mag;nification the nature of the constituents was not resolved. 
O o 
