172 PROF. W. J. SOLLAS ON A PROCESS FOR [May 1902, 
Herein lies a great field of work. At present a really satisfactory 
beginning has been made in the case of the felspars alone ; and it is 
fortunate for this method that these minerals enter so largely into 
the composition of most igneous rocks. But with the great advance 
which has marked of late our processes of investigation, a 
rapid progress may be looked for in this direction. All that is 
required is that the chemist and mineraiogist should work more in 
union: many good analyses are practically worthless, because the 
mineral of which they have been made has escaped a physical 
investigation; and many physical investigations are shorn of 
half their value, because they have been made on material, the 
chemical composition of which has been imperfectly determined, 
if at all. 
After reaching this stage in our enquiry, it appeared to me 
necessary to investigate more closely the degree of exactitude with 
which a mineral separation may be accomplished. I have conse- 
quently made some experiments which may contribute towards a 
solution of this question. 
The impalpable powder or ‘ flour,’ which is necessarily produced 
from a rock in the process of comminution, may obviously prove a 
serious source of error, since, becoming entangled among the 
larger particles or grains, it may retain a position in the separator 
altogether independent of its specific gravity. To remove this flour 
might tend to increased accuracy, and would certainly shorten the 
time required for subsequent operations; but naturally, before 
actually venturing on such a step, we must first ascertain whether 
the amount of flour in relation to that of grains is constant for 
different minerals, or, still more important, whether any process 
that we may adopt for the removal of the flour may not carry away 
different amounts of it in different cases. To determine this, 2 or 
3 grammes of a mineral were crushed in a mortar, passed through a 
fine sieve, and a weighed quantity placed in a wide beaker; water 
was added to a depth of 10 mm., the mixture stirred and allowed 
to stand for 15 seconds; the supernatant fluid was then poured off 
onto a filter, and the process repeated until at the expiration of 
15 seconds the water remained clear. The grains remaining in 
the beaker were washed onto a filter, dried, and weighed, as also 
was the flour that had been removed. In this way the quantity of 
flour separated by washing was directly ascertained. The following 
are the results for a few important rock-forming minerals :— 
Per cent. 
Orthoclase: s.5on28 22°58 to 22°94 
Owarttc, ei. s an caer 20°94 to 24:20 
Hornblende....... vee 16°27 to 16°73 
Augite 25.5002 eee 15°56 
IBIOLIbG! 05s eb eee 19°50 
From an examination of this list it would appear that the amount 
? 
