48 SCIENCE PROGRESS 
zircon, rutile, monazite, sillimanite, and quartz. The result of 
the magnetic separation was as follows: 
Per cent. 
Magnetite . : = . about 1°3. 
ae \ : ; ; > »  70°5 (only a trace of garnet). 
Monazite . s : ; = 5°6. 
Rutile 
Zi : é 
Seem »  22°5 (chiefly zircon). 
Quartz 
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 
——— Le ee 
