28 



Dr. F. T. Trouton. An Experimental [Nov. 21, 



been suggested as a measure of the qualities of a substance referred 

 to in a collective way under the general name of hardness. Any 

 comparative method, such as " Mohr's Scale," even though it be 

 pre-eminently suitable for practical purposes, must be considered 

 as objectionable, from a physical point of view. This will be under- 

 stood, if it be attempted to define or measure the property in question 

 for any substance, supposing no other kind of matter to exist. 



In Table I are given the coefficients of attrition in the case of seven 

 substances. These numbers, when multiplied by 10 -12 , are in terms 

 of C.Gr.S. units, that is to say, they are the weights of material in 

 grains, attrited per square centimeter, during a centimeter displace- 

 ment under a pressure of one dyne per square centimeter. To bring 

 these to English measure (any mass, feet and seconds), we need not 

 trouble about the mass, but have only to multiply twice by 30'48, the 

 number of centimeters in a foot, since the dimensions of attrition, it 

 will be remembered, are the same as that of the inverse square of a 

 velocity. 



Table I. 



Attritivity x 10~ 12 

 in C.Gr.S. units. 



Quartz 4*0 



English plate glass. ... 5' 7 



French plate glass .... 8*8 



Serpentine 22*0 



Marble 100*0 



Alabaster 175*0 



Plaster of Paris 599*0 



The quartz cylinders used were cut with the axis parallel to the 

 principal axis of the crystal ; the material looked transparent and 

 clear. The English plate glass had the usual greenish appearance. 

 The French plate was white and clear. The serpentine was the 

 variety known as Galway marble — olive-green with whitish streaks. 

 The marble cylinders were made of white statuary marble, and 

 contained disseminated throughout its mass small and transparent 

 microscopic crystals of a felspar, probably albite. The alabaster 

 was whitish, with a slight flesh-coloured tinge ; it was rather inclined 

 to wear into somewhat roundish lumps, due, no doubt, to want of 

 uniformity. The cylinders of plaster of Paris were cast in moulds, 

 and, after having thoroughly set, were turned up in a lathe. 



The cylinders were all about 5 cm. in diameter, and varied in 

 length from 0*5 cm. to 4 cm., according to the amount of material 

 available. 



Particular care has to be taken to thoroughly remove the attrided 

 material. If a fine dust is allowed to remain attached to the cylin- 



