THE ELECTROMAGNET IN PETROGRAPHY 39 



In this memoir Fouque described what he called a new 

 mechanical process, by means of which he had been able to 

 isolate the felspars of the Santorin lavas. In this process 

 the rock was crushed and the crushings submitted to the 

 action of a strong electromagnet. The iron ores, the ferro- 

 magnesian minerals, and the partially differentiated ground 

 of the rock were extracted, leaving a non-magnetic residue of 

 felspar. The apparatus used by Fouque consisted of an electro- 

 magnet put into action by a Bunsen battery, and was essentially 

 similar to the apparatus used by Pouillet, Delesse, and others. 

 Fouque also appears to have used a dynamo in some cases, 

 as a substitute for a Bunsen battery. His results are very 

 interesting, and prove convincingly the importance of the 

 electromagnet as a means of isolating minerals. By its aid, 

 and in conjunction with other methods, he was able to demon- 

 strate that a given specimen of lava might contain quite a 

 medley of plagioclases. He also proved that two varieties of 

 pyroxene were present, both rich in lime, but differing con- 

 siderably in the proportions of magnesia, and ferrous oxide ; 

 also two olivines which differed in the amount of ferrous oxide 

 present. 



It is quite clear, however, from the description which has 

 been given of the work done by Delesse and others, that 

 Fouque, using the electromagnet in his researches, was using 

 an old and not a new process. It seems scarcely credible that 

 Fouque could have been unaware of the work already done in 

 this field by his fellow-countryman Delesse, and published in a 

 journal so well known as the Annates des Mines. Even in the 

 Mineralogie micrographique of 1879, in which Delesse's other 

 work is fully recognised, Fouques error remained unrepaired, 

 and it has unfortunately been persisted in by later text-book 

 writers. 



Doelter, 1 following Fouque, and apparently unaware of any 

 important results other than those which the latter obtained, 

 applied the electromagnet to the study of rocks and minerals. 

 He wrongly attributes to Fouque the priority for using an 

 electromagnet in the systematic study of weakly magnetic 

 rock-forming minerals. He makes matters worse by referring 



1 " Ueber die Einwirkung des Electromagneten auf verschieden Mineralien 

 und seine Anwendung behufs mechanischer Trennung derselben," Sitzungberichte, 

 Wien, 1882, Band lxxxv. Abteilung i. 



