DIFFERENTIAL PRESSURE ON MINERALS AND ROCKS 497 



whose capabilities in this direction are as yet unknown and then to 

 still harder minerals which are considered to be perfectly brittle, so 

 far as this property is known to be possessed by any body in perfection. 

 The mineral species chosen were for the most part those constituting 

 Mohs's Scale of Hardness, although certain others were also included 

 in the list. The series was as follows: 



Name of Mineral Hardness Mohs's Scale 



Selenite 2 Limonite 5-6 



Rock salt 2.5 Orthoclase 6 



Iceland spar 3 Magnetite 5 . 5-6 . 5 



Fluorite 4 Pyrite 6-6 . 5 



Apatite 5 Quartz 7 



Diopside 5.5 Garnet 6 . 5-7 . 5 



The same body would undoubtedly give different values for 

 plasticity if tested in different ways, just as the same body gives 

 different values for the breaking point, according to whether the 

 latter is measured by bending, tension, or impact. 



Speaking generally, however, under ordinary conditions of tem- 

 perature, hardness is a function of plasticity and minerals become less 

 plastic as they become harder. 



'■ Selenite. — A clear transparent crystal from Ellsworth, Ohio, was 

 selected. It was perfect in form, being bounded by the prismatic 

 faces in combination with the clinopinacoids and clinodomes. The 

 crystal measured 1.246 inches (31.6 mm.) in the direction of the 

 vertical axis, and 0.618 inch (15.68mm.) in the direction of the 

 ortho-diagonal axis. This was placed in a copper tube 1.75 inches 

 (44.45 mm-) high and otherwise of the standard size commonly 

 employed in these experiments, namely, having an internal diameter 

 of 1 .062 (ly^ inches, or 26.98 mm.), the walls of the tube being o. 125 

 (I inch or 3 . 175 mm.) thick. 



The crystal was placed in the tube on end, as shown in Plate I, 

 Fig. a, so that it rested on the lower solid angle, the line of the inter- 

 section of the clinodomes being inclined at a considerable angle to 

 the plane of the end of the inclosing tube. Paraffine wax was used 

 as an embedding material, since alum, sulphur, or fusible metal melts 

 at temperatures above that at which selenite loses its water. The 

 paraffine wax, melting at a temperature below that of boiling water. 



