66 FIELD MUSEUM OF NATURAL HISTORY 
Hall 78. 
SALTS OF THE ALKALIES AND ALKALINE 
EARTHS. 
These include besides salt and similar compounds such min- 
erals as asbestos and mica. Here belong also the borates, phos- 
phates, etc., which occur in nature chiefly in combination with 
lime or soda. Besides the alkali and alkali-earth compounds, 
sulphur and abrasives may be found in this hall as well as var- 
ious minerals of special or limited use. 
The series of abrasive and polishing materials exhibited in- 
cludes specimens of corundum, emery, garnet, and quartz. 
These are the more important minerals used for rapid grinding 
and are illustrated by specimens from many localities. The 
rocks and minerals associated with the emery are characteristic 
and should be examined in connection with the emery itself. 
For more delicate work and for polishing are shown speci- 
mens of pumice, tripoli, siliceous chalk, and similar materials, 
which are in general softer than the rapid grinding materials. 
Agate, hematite, and flint are shown as examples of burnish- 
ing materials used to produce a very high polish on metallic 
surfaces. 
Whetstones are represented by specimens of great variety 
in mineralogical composition ranging from a siliceous, fine- 
grained shale to gneiss and mica schist. 
The great variety of substances adapted to grinding and pol- 
ishing is well illustrated here. 
Artificial abrasives are represented by carborundum, artifi- 
cial corundum, and several substances of lesser importance. 
Borax minerals and products form one large series. The 
minerals shown are borates of lime and soda. Other boron 
compounds such as tourmaline, while of fairly common occur- 
rence, are not sufficiently rich in borax to be of economic value 
as a source of this material and do not appear in this series. 
Boron compounds of use in the arts and in medicine are illus- 
trated by boric acid and the several forms of borate of soda or 
common borax. A boride of iron shown is representative of a 
series of boron compounds of recent introduction to the metal- 
lurgical industry. 
A large series of the compounds of the alkaline earths, that 
