5 o SCIENCE PROGRESS 



from 300 grams' of which he obtained 0*022 gram of zircon — 

 i.e. about o'ooy per cent. The method used by Chrustschoff, 1 

 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," Tschermak's Mittheilungen, 

 Band vii. 1886. 



