DETERMINATION OF MINERALS. 379 
IV. DETERMINATION OF MINERALS. 
In the determination of minerals, no one order in the 
succession in which characters should be examined answers 
for all minerals, or even for all of the same section of species. 
A. For species having a metallic lustre : 
Color will be first noted ; and then streak—that is, the 
color of the mineral on a surface scratched or abraded by a 
fine file, or when very finely powdered, and the lustre of the 
powder or abraded surface, whether metallic hke the min- 
eral or unmetallic. Hardness should be ascertained when 
obtaining the streak. 
Blowpipe and chemical characters are of the highest value, 
giving generally the most certain results. 
Specific gravity 1s especially distinctive with species hay- 
ing a metallic lustre, since the differences in density among 
such species are usually large.* 
Crystalline form and cleavage are of first importance, 
whenever the specimen allows of their determination. 
B. For species without metallic lustre : 
Streak is sometimes of importance, especially among spe- 
cies in which it is highly colored. 
Color is generally of little value owing to the variations 
that frequently come in through impurities. 
Lustre is one of the first characters the eye will observe, 
but its variation under most species is wide, and often it 
is of little value. State of aggregation and fracture for the 
most part serve to distinguish only varieties. 
Hardness is also often a Vv: arying character, the range 
under some species being from 1 to 6 in the scale of hard- 
ness ; and still its indications are generally important. 
Crystalline form and cleavage are always important when 
observable. 


* In using the spiral balance of Jolly (page 65), the spiral spring is put at any de- 
sired height by means of the sliding rod C. The stand B is raised so that the lower 
pan, d, shall be in the w ater, while ‘the other, ¢, is above it. The position of the_n- 
d -x, or signal, 7, ix then noted, by sighting across it and observi ing that thei: d x 
and the image of itin the mirror are in the same horizontal line ; let s stand for iv. 
Next put the fragment of the mineral in ¢, and drop the stand B until the lower pai 
hangs free in the water, and note the position of m, which we may represent by ¢; 
(—s will equal the weight i in the air. Now place the fragment in the lower pan and 
after adjusting again “the stand B, the position of m is noted as before ; call it wv. 
See beh = loss of w eight in water. From these values the specific grav ity isat once 
obtaine 
