42 SCIENCE PROGRESS 
compact types of rocks. The method can only be trusted 
quantitatively when applied to holocrystalline rocks, in which 
the particles obtained by crushing consist of individual (ze. 
unmixed) minerals. In fairness to Doelter, however, it should 
be admitted that he was aware of the fact that in the case of 
hemicrystalline rocks the results were somewhat unsatisfactory. 
But in his enthusiasm for the method he went so far as to claim 
even for these, that they were of value as controlling and 
supplementing the results of chemical analysis. This claim 
may be fully respected when applied to holocrystalline rocks, 
but its assertion in the case of hemicrystallines is by no means 
warrantable. 
The conclusions to which Doelter was led by the results 
of his experiments with the electromagnet summarise in an 
admirable way the scope of that instrument’s utility as applied 
to rocks. His conclusions were: (1) that the electromagnet is 
of great service in petrographical studies; (2) that minerals 
can often be isolated by its aid which cannot be conveniently 
isolated by any other means; (3) that it is especially useful 
when applied in conjunction with the use of heavy liquids; 
(4) that the separations made by it, even when incomplete, are 
generally of great use, while in some cases they may be taken 
as being of somewhat definite quantitative value. 
In a criticism of Doelter’s work, Pebal! called attention to 
the fact that, when operating on fine crushings, the separation 
can be made more efficient by keeping the crushed fragments 
immersed in water during the process. Doelter attempted to 
refute this recommendation made by Pebal; but a comparison 
of experimental results indicates that the recommendation is a 
very good one, and that its adoption substantially increases 
the accuracy of the results for the finer crushings. 
Mann? used the electromagnet to isolate augite for analysis 
from certain phonolites. He followed Pebal in the use of the 
wet method of separation, and denounced the dry method of 
Fouqué and Doelter. Mann also adopted a novel procedure 
in making some of his separations. He placed his magnet 
horizontally, and allowed the mineral fragments to fall in water 
! Sitzungberichte, Wien, 1882, Band lxxxv. Abteilung i. 
* “Untersuchungen iiber die chemische Zusammensetzung einiger Augit aus 
Phonolithen und verwandten Gesteinen,” Mewes Jahrbuch fur Mineralogie, etc., 
1884, Band ii. 
