Polytechnic Association. 619 



The conversion of the chromic iron into the yellow or neutral chro- 

 rnate of potassa, and thence into the chromic acid and the green chromic 

 oxide, may be shortly stated. The chromic iron is finely pulverized 

 and calcined, and then mixed with powdered slacked lime intimately, 

 both of which are now stirred in a hot concentrated solution of sul- 

 phate of potassa, so as to form a paste, which is then stamped into 

 wooden cylindrical moulds, and exposed to a powerful oxidizing flame. 

 The greater part of the chromic oxide is converted into chromic acid ; 

 the whole mass is then broken up and thrown into boiling water, and 

 left to crystallize. The yellow crystals are formed in oblique four- 

 sided pyramids ; are of such intense color that one part of it dissolved 

 in 40,000 parts of water will retain a yellow tint. It has an alkaline 

 reaction, is permanent in the air, of bitter taste, and very soluble in 

 boiling water, but insoluble in alcohol. 



The bichromate of potassa is obtained by adding sulphuric acid to 

 the above solution of the yellow chromate while hot, and on cooling 

 intense orange-red crystals are soon formed, which are rectangular 

 prisms ; they are not altered by exposure to the air ; are soluble in 

 ten parts of water, but insoluble in alcohol. 



In order to obtain the sesqui or green oxide of chrome, we have 

 only to precipitate the bichromate solution by ammonia. This green 

 oxide is principally used as a pigment for porcelain or glass, to produce 

 an emerald-green color. A simpler method of producing a fine ses- 

 quioxide is by igniting twenty parts of bichromate of potassa with 

 four parts of sulphur for half an hour. By mixing the yellow 

 chromate of lead and Prussian blue together, a beautiful green pig- 

 ment is obtained. 



The neutral salts of chrome form yellow precipitates, while the 

 basic salts produce orange precipitates. It is wonderful to see how 

 many pigments are obtained from chromium ; for instance, a brown 

 oxide is obtained by. digesting the green oxide with chromic acid ; 

 another yellow precipitate is had from a combination with zinc salt, 

 and a yellowish-brown pigment from sulphate of copper and the 

 chromates. 



Peach-purple crystals are obtained from the sesquichloride of chrome. 

 Chrome-alum obtained from the combination of sulphates of chromium 

 and ammonia, in the form of deep purple crystals, is very exten- 

 sively used by calico printers. 



Dr. P. H. Yan der Weyde — A. more interesting subject than chro- 

 mium could not have been selected ; for there is no more wonderful 

 substance in the whole field of chemistry. It has been found that 



