Polytechnic Association. 617 



Suspension Bridge. 



Mr. Dudley Blanchard exhibited a model of a suspension bridge, 

 intended to illustrate the strain upon the several parts, and explained 

 its theory. He had tested the strain at different points by introducing 

 a system of levers to measure it. 



Adjourned. 



October 19, 1871. 



Prof. S. D. Tillman in the chair ; Robert Weir, Secretary. 



Chromium and its Compounds in the Arts and Medicines. 



By Dr. Lewis Feuchtwanger. 



[Illustrated by many natural and artificial productions.] 



This metallic element is of a very remarkable nature, although its 

 compounds do not occur very widely distributed over this globe. 

 Chromic iron is the only native ore from which all the compounds 

 are produced that form important vehicles in the arts, and as pig- 

 ments of very bright colors. The metal is one of the most infusible 

 substances — much more so than platinum. It combines with oxygen* 

 forming only four different oxides, the fourth oxidation being an acid, 

 and when combined with three atoms of oxygen is isomorphous with 

 the magnetic oxide of iron. 



Chromic iron is found only in serpentine rocks, forming veins and 

 imbedded masses, or in grains mixed with sand. It is quite abund- 

 ant in Siberia, Styria, the Shetland Islands, Cuba — the United States 

 furnishing by far the largest number of localities. The writer exam- 

 ined large deposits in California, Maryland, Pennsylvania, Vermont 

 and North Carolina. Its component parts are 60 per cent oxide of 

 chromium, 20 per cent protoxide of iron, 12 per cent of alumina, and 7 

 per cent of magnesia. 



Another native mineral, called Crocoisite, occurs in Siberia, resem- 

 bles the artificial orange chrome-yellow of the shops, and is, in fact, a 

 chromate of lead, Pb Cr 4 . There is another, but rare mineral, 

 called Vauquelinite, which is a chromate of lead and copper, found in 

 Brazil, which Dr. Torrey found fifty years ago in the lead mines at 

 Sing Sing, N. Y. 



The principal uses to which the chromic compounds have been put, 

 are : — 



1. The yellow or neutral chromate of potassa ; it is the base for all 

 other salts, being prepared by a simple process from the chromic iron. 



