664 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1907. 
Particles of subway dust, not iron, comprised bits of silica, cement, 
stone, fibers of wood, wool and cotton, molds, and undistinguishable 
fragments of refuse of many kinds. 
Besides the dust which resulted from the grinding of metals, it 
was evident that the gradual Wear and tear of many substances in the 
subway contributed to the dust. 
Chemical composition of the dust.—The separate chemical analyses 
of eleven samples of accumulated dust from the subway showed the 
following average percentage composition: Total iron, 61.30, includ- 
ing 59.89 metallic iron; silica, etc., 15.58; oil, 1.18; organic matter, 
21.94, as shown in fig. 10. 
ote, 
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SOOM eS 
SAINI 
Fic. 10.—Composition of subway dust as determined by chemical analyses 
of eleven samples. 
Origin of metallic dust——A large part of the metallic iron came 
from the wear of the brake shoes upon the steel rims of the wheels of 
the cars. 
The wear upon the brake shoes was very severe. By weighing 
them when they were new and after they were worn out, and deter- 
mining the number used, it was calculated by the operating company 
that one ton of brake shoes was ground up every month for each mile 
of subway. The brake shoes consisted of cast iron with steel inserts. 
There was also some loss to the rails and rims of the wheels and to 
the contact shoes which ran upon the third rail. Probably 25 tons per 
month would be a low estimate of the weight of iron and steel ground 
up in the whole subway every month. 
