4 8 SCIENCE PROGRESS 



zircon, rutile, monazite, sillimanite, and quartz. The result of 

 the magnetic separation was as follows : 



In this case the monazite was almost completely isolated from 

 the other ingredients, and the chief object of the separation was 

 to determine its amount quantitatively, so as to get an approxi- 

 mate estimate for the amount of thoria in the total concentrate. 

 In such an instance, when once the amount of thoria peculiar 

 to the monazite grains of a district has been determined by 

 chemical analysis, it becomes an easy matter to estimate the 

 approximate thoria percentage of a concentrate by magnetic 

 separation alone. Such estimates usually agree fairly closely 

 with the results of chemical analysis. 



Even in the absence of exact quantitative results for each 

 mineral, the separation of the sand into groups of constituents 

 gives a very clear idea of its mineral composition, and greatly 

 facilitates examination with the microscope. 



Another example of the usefulness of the electromagnet, 

 especially when used in conjunction with other methods, may be 

 taken from its application to a crushed rock. It is an especially 

 useful means of isolating, or observing the presence of, certain 

 accessory minerals in small quantities, such as apatite, rutile, 

 zircon, and pyrite. The rock in question was an olivine gabbro ; 

 it was partially ophitic and medium-grained in texture. Twenty 

 grams of the crushings were treated with the hand magnet to 

 extract the magnetite. The remainder was treated with the 

 electromagnet in a fairly strong field and divided into two parts : 

 the magnetic portion was found, on examination with the micro- 

 scope, to consist of a mixture of pyroxene (partly rhombic), 

 olivine, hornblende, and brown biotite ; the non-magnetic portion 

 consisted almost entirely of plagioclase, with a little apatite and 

 zircon, together with traces of pyrite and quartz. The separation 



