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 {i.e. 

 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 : (i) 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 l 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 2 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 

 Fouque 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 



1 Sitzungberichte, Wien, 1882, Band lxxxv. Abteilung i. 



3 " Untersuchungen iiber die chemische Zusammensetzung einiger Augit aus 

 Phonolithen und verwandten Gesteinen," Neues Jahrbuch fiir Mineralogie, etc., 

 1884, Band ii. 



