68 FIELD MUSEUM OF NATURAL HISTORY 
Native alum from various localities and alunite from which 
alum is extracted represent another class of minerals of some 
economic importance. 
Nitrates are represented by several varieties of the soda nitre 
of Chili and by cave earths from caves in various parts of the 
world. 
Mica is represented by a series of specimens mainly from 
American and Russian quarries. Besides thin plates of mica 
of the best quality specimens of low grade material enable the 
visitor to note the difference between the ordinary micas and the 
more valuable kinds. The rocks in which mica occurs are also 
shown. 
Asbestus is represented by two series, one of crude mineral 
and one illustrating its uses. These specimens are of asbestus as 
mined and are not confined to the commercially valuable ma- 
terial. Consequently all stages between a merely compact but 
somewhat columnar tremolite and the finely fibrous, flexible 
asbestus of the best quality may be traced through these speci- 
mens. 
Phosphates are represented by a large series of specimens 
classified as Mineral phosphates, Rock phosphates, and Guanos. 
The principal localities represented are Tennessee, Carolina, 
Florida, Russia, Spain, Venezuela, and Cuba. 
Gypsum is represented by a separate collection of specimens 
from many parts of the world, including the coarser kinds mined 
for land plaster, plaster-of-Paris, wall plaster, etc., and the finer 
varieties, selenite, alabaster, and satin spar, quarried for orna- 
mental purposes. 
Native sulphur and those metallic sulphides from which 
sulphur is regularly extracted form a single collection. With 
these specimens is a series illustrating the various forms in which 
sulphur reaches the market. 
Processes for the manufacture of soda are illustrated by 
three collections. One shows the Leblanc process, all stages of 
which are illustrated by specimens of the raw, intermediate, and 
final products. The relations of the various products to each 
other are indicated by connecting lines. 
The ammonia process is fully illustrated by a series of the 
raw materials, intermediate and final products being so ar- 
ranged that the process may be followed step by step. The 
electrolytic process is illustrated in the same manner. 
