No. 377-| REVIEWS OF RECENT LITERATURE. 377 
poses of final identification. As far as possible only those chemical 
reactions are described that are necessary to identify the minerals, 
and these are always simple ones. 
The book is too well known to all mineralogists to require further 
characterization. It is sufficient praise to state that the fourth edi- 
tion is an advance over all the editions that have preceded it. 
S. B. 
Hardness of Minerals. —Jaggar has described! a new instru- 
ment for the determination of the hardness of minerals. After 
briefly summarizing the results of previous workers on this subject, 
he describes the chief sources of error in their methods as follows : 
“ (1) personal variability due to using ‘visibility’ of a scratch as 
determinant ; (2) inequalities of surface; (3) undefined details of 
instrument. To eliminate (1) the depth a abrasion should be defi- 
nite and measurable ; to eliminate (2) the surface should be artificial 
and defined, and the boring method, where only a very small portion 
of the surface is initially touched, should be used ; to eliminate (3) 
every part of the instrument, including the abrader, should be 
minutely defined, and for comparative determination an ae 
standard should be fixed.” 
The instrument devised to overcome these difficulties sae to meet 
the other conditions of the problem presented is intended to be 
applied to the microscope in order that the measurements may be 
made either on a crystal face or the surface of a thin section, that 
the test may be applied to very small portions of mineral, and that 
the control of the instrument may be very exact. ‘The principle of 
the instrument is as follows: a diamond point of known, constant 
dimensions is rotated on an oriented mineral surface, under uniform 
rate of rotation and uniform weight, to a uniform depth. The num- 
ber of rotations of the point, a measure of the duration of the abra- 
sion, 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 with various adjust- 
ents. 
(2) A balance beam and its yoke. 
(3) A rotary diamond in the end of the beam. 
(4) Apparatus for rotating uniformly. 
1 Jaggar, T. A., Jr. A Microsclerometer for Determining the Hardness of 
Minerals. Am. Jour. of Sci., vol. iv, pp. 400-412, 1897. 
Also Zeit. f. Krystall., vol. xxix, pp. 262-275, 1898. 
