and Laboratory Methods. 2659 



Fig. 7, and displays a slender wedge, with nearly plane faces, with terminal 

 angle about 12°. 



Hone W. In this case, while the main faces of the wedge are inclined 

 toward each other at an angle little exceeding that above stated, with Hone A, 

 they terminate in two small faces which meet at the edge-line at about 18°; this is 

 the low wedge already distinguished on the vertical and horizontal sections. 

 Sometimes a third set of planes occur ; so that this cross section differs from 

 that obtained with Hone A in the presence of inclined shoulders and in a some- 

 what larger terminal angle. 



Hone I. The cross section of the wedge shows three or more irregular 

 changes in the inclination of its faces as they approach the edge. The edge 

 itself is rudely rounded, though sometimes nearly or quite flat, and the mutual 

 inclination of the nearest planes may be 22°, in some places much more. 



It is apparent that the angle obtained in honing should be fitted to the kind 

 of tool and work in view ; it may be large, as in a woodman's axe, or minute, as 

 in the surgeon's scalpel or the knife of the microtomist. The advantage of 

 strength, which accompanies a wide angle and series of shoulders, can be sacri" 

 freed only in the case of delicate tools of small angle, adapted for use with 

 drawing motion, with little application of force, for penetration of soft 

 tissues. 



I am indebted to Mr. G. B. Waterhouse of the Department of Metallurgy, 

 Columbia University, for the following pertinent facts concerning the micro- 

 scopic structure and constitution of steel; and for an examination of the steel in 

 these chisels : 



" Tool steels, which contain usually over 1 per cent, of carbon, present on 

 fracture a crystalline condition, the individual particles consisting of pearlite, 

 covered by a thin brilliant film of cementite, that has been found to possess the 

 definite formula FegC. When a surface of such a steel is carefully ground, 

 polished, and lightly etched with dilute nitric acid, it exhibits under the micro- 

 scope a thin meshwork of brilliant white cementite enclosing the dark pearlite 

 areas. A homely comparison is the honey-comb, the cell-walls being cementite, 

 and the cells of honey pearlite. 



" Under a high magnification power the pearlite is seen to be composite in its 

 nature, a condition exceedingly w-ell shown by steels that have been very slowly 

 cooled from a bright red heat. This consists of an aggregate of thin plates of 

 the definite carbide, cementite, interstratified with metallic iron. 



" However, in the process of hardening, as the steels are raised past a dull 

 red, at 6sO°C, the change or rccakscoicc point, the pearlite areas become homo- 

 geneous, and form a new constituent, to which the name of hardenite has been 

 given. To this constituent the intense hardness of quenched steels is due. As 

 the heating is continued the cellular structure disappears, the cementite and har- 

 denite diffusing one into the other ; but after quenching in cold water it is found, 

 in most cases, that a little cementite has again formed into meshes, notwithstand- 

 ing the rapid cooling. 



" Steels in this condition are intensely hard, being equal to No. 7, quartz, in 

 Moh's scale, and are also very brittle. It is therefore the custom before use to 



