determining the Hardness of Minerals. 403 



instrument, including the abrader, should be minutely defined, 

 and, for comparative determinations, an empirical standard 

 should be adopted. 



The Microsclerometer. 



The author's object in the present research is to describe an 

 instrument so precisely adjusted as to eliminate these earlier 

 sources of error. The quality which it is proposed to measure 

 is the resistance opposed by a body to the removal of particles 

 of its substance by a defined diamond point, moving in con- 

 tact with it under uniform conditions. The instrument is 

 applied to the microscope, so that it may be used for either 

 thin sections or crystal faces ; it is believed that the tests 

 described will eventually be of determinative value in petrog- 

 raphy. The adjustments of the instrument are such that any 

 of the variable elements in the process of abrasion may be 

 made functional while the others are maintained constant ; but 

 the best movement for obtaining a mean value is the rotary 

 movement described by Pfaff, the number of rotations of the 

 boring point indicating the hardness of the substance relative 

 to the abrading point. 



The instrument is shown in Plate XII, adjusted to the large 

 Fuess microscope No. 1. Fig. 1 shows the plan, and fig. 2 the 

 vertical elevation. The principle of the instrument is as fol- 

 lows: A diamond point of constant dimensions is rotated on 

 an oriented mineral section under uniform rate of rotation and 

 uniform weight to a uniform depth. The number of rotations 

 of the point, a measure of the duration of the abrasion, varies 

 as the resistance of the mineral to abrasion by diamond : this is 

 the property measured. The instrument consists of the fol- 

 lowing parts : 



(1) A standard and apparatus for adjusting to microscope. 



(a) Foot adjustments, (b) Rotating adjustments, 

 (c) Lifting adjustments, (d) Fixing adjustments. 



(2) A balance beam and its yoke. 



(3) A rotary diamond in its end. 



(4) Apparatus for rotating uniformly. 



(5) " " recording rotations. 



(6) " " locking and releasing. 



(7) " " recording depth. 



The instrument described admits of measurement with any 

 one of the four variables, rate, weight, depth or duration. 

 The last has been found most practical, because it gives the 

 highest values and hence admits of the most delicate gradation. 

 The Standard. — The yoke y is supported on a brass column, 

 which slides in an outer tube, and may be raised or lowered by 



