November 20, 19 19] 



NATURE 



m 



an angle of about 90°. Some surface light is visible 

 at d and i. At low pressures the dark cones c and 

 a merge softly into h. As the pressure is increased, 

 the interfaces become more intense and clearly de- 

 fined, but the angles do not appreciably alter. The 

 central cone proceeds from the surface first as a rod 

 or filament of a remarkably black intensity. 



Further gentle increase of pressure causes the sur- 

 face layer to rupture as indicated in Fig. 2, which is 



a photograph of a surface repeatedly ruptured by gentle 

 impact. If the Nicols are paralleled, black rays will 

 be seen proceeding from the edge of the crack as in 

 Fig. 3, their direction indicating that the crack is 

 normal to the surface and merely superficial. The 

 Fig. I appearance remains unaltered. 



A new phenomenon makes its appearance when the 

 pressure is again increased. Immediately under the 

 ball there appears, as in Fig. 4, a sphere pier.ced by 

 the filament of the cone a, and having a bjack outline 

 tinged with red on the outside. The interior is filled 

 with green-blue light, otherwise the general appearance 

 of Fig. I remains unaltered. If now the Nicols are 

 paralleled, the conical fracture (Fig. 5) previously 

 illustrated by Prof. Raman will be seen, and by 

 examination at intermediate positions of the Nicols, 

 it will be evident that the cone fracture which takes 

 place along the surface of b is tangential to 

 the sphere which it encloses. The fracture can be 

 extended up to a limit which it is difficult to exceed, 

 even by a great increase of the rate of application of 

 the pressure. 



If the pressure is increased again, the crushing point 

 is soon reached. The glass under the ball collapses 

 almost explosively, a faint click being audible, and the 

 ball sinks deeply through the surface. The cavity thus 

 produced is lined with a snow-white layer of powdered 

 glass. 



On the polariscope at the moment of fracture several 

 interesting developments may be observed. First, 

 as the result probably of the greater area of pressure 

 contact, the cone of light h broadens out laterally; 

 secondly, the cone fracture may extend horizontally 

 like the brim of a hat, thus definitely terminating the 

 depth below the surface; and thirdly, the space within 

 the cone becomes cleft bv two fracture planes appar- 

 ently normal to one another, and having their line of 

 intersection on the axis of the dark cone a. 



If only one diametral plane appears, the other may 

 be developed by an increase of pressure. Sometimes 

 the second plane is terminated at the axis, but it can 

 always be extended across the first plane. The ap- 

 pearance of the fractures is now as indicated in Fig. 6. 

 If the diametral plane fractures terminate at the base 



NO. 2612, VOL. 104] 



of the cone, they may be extended by pressure to the 

 end of the rim, as in Fig. 7. 



Under crossed Nicols two new coloured spheres identi. 

 cal with the original one may make their appearance 

 just under the base of the cone fracture, as in Fig. 8, 

 indicating the existence of subsidiary fractures. 



But if a polished transverse section of a glass plate 

 smoothed with carborundum is observed microscopic- 

 ally, it will be seen immediately that surface conchoidal 

 fractures predominate, and 

 that, if cone fractures do exist, 

 they are very shallow. 



In the workshop process of 

 smoothing preparatory to 

 polishing, the smcjothing tool 

 moves over the glass surface, 

 but movement of the experi- 

 mental plates did not alter the 

 general characteristics outlined 

 above, nor did the presence of 

 water afford an explanation of 

 the results obtained in prac- 

 tice. 



Evidently mere pounding of 

 a glass plate cannot result in 

 a smoothed surface of a tech- 

 nical order. 

 Fia.io. .As in all the previous experi- 



ments the pressure was applied 

 at the centre of the block, 

 where the horizontal forces were balanced, a new 

 series was carried out near the margin of the plate. 

 The new appearance, corresponding with the stage 

 illustrated in Fig. i, is indicated in Fig. 9, frorri 

 which it will be seen that the central cone is now 

 deviated towards the side, its axis following the char- 

 acteristic conchoidal section. In other respects the 

 sequence of phenomena was as before. Thus, after 

 the cone fracture which was of a shallower order took 

 place, and the crushing point was reached, the dia- 



Pl^fTe dijmerral fractvrvs 

 Fw. 6. 



tool. 



g/JSS 



Fig. 2. — Surface percussion cracks on gla&s. Magnification 43 X . 



metral plane fracture followed the axis of the deviated 

 central cone, and the cavity from which the splinter 

 was removed had the characteristic conchoidal ap- 

 pearance. 



It is presumably the impact of the carborundum 

 grains on the edges of cavities on the glass to be ground 

 that produces the conchoidal splinters as indicated in 

 Fig. 10. It would appear, therefore, that it is the 

 diametral plane fracture that is of priman.' importance, 



