50 SCIENCE*PROGRESS 
from 300 grams of which he obtained o'022 gram of zircon— 
2.é, about 07007 per cent. The method used by Chrustschoff,} 
however, lacks the neatness and precision which is attainable by 
the use of the electromagnet in combination with the use of 
heavy liquids. 
6. CONCLUSION 
To enter at further length on a description of the magnetic 
treatment of rock-forming minerals would unduly prolong this 
article. Sufficient has been stated to indicate that, to students 
of petrography, the electromagnet is an instrument of consider- 
able value. It only remains to point out that, like every other 
means of observation, its sphere of usefulness is a limited one, 
and too much should not be expected from it. The presence of 
magnetic impurities in otherwise non-magnetic minerals is 
obviously a disturbing factor of great importance. For this 
reason, the use of the magnet should in all cases be controlled 
by examination with the microscope, and no separation should 
be taken as being exact, without confirming the purity of 
the separated portions in this way. Used in this manner, 
the method of magnetic separation is one which gives great 
assistance aS a mode of physical rock analysis. Those who 
are anxious to make themselves in some measure independent 
of rock sections, and to avail themselves of the many advantages 
offered by a systematic study of crushed rocks, will find it a 
most useful adjunct in a somewhat neglected field of work. 
1 “ Beitrag zur Kentniss der Zirkone in Gesteinen,” 7schermak’s Mittheilungen, 
Band vii. 1886. 
