424 PROCEEDINGS OF THE AMERICAN ACADEMY. 



of any sort but with small holes, in order to remove particles of silver 

 dichromate either suspended in the solution or floating on the surface. 

 These particles were always of considerable size, so that the resulting 

 solution was clear. After the saturated solution had cooled and had 

 deposited the greater part of its charge of salt, the mother liquor was 

 continuously used to dissolve fresh portions of salt. About one liter 

 of acid was used for the crystallization of about fifty grams of dichro- 

 mate. Although by this method the impurities in the original salt 

 accumulate in the mother liquor, on account of the relatively large 

 volume of the mother liquor, there was little danger of these impurities 

 being carried into the second crop of crystals. It was shown, for in- 

 stance, that the mother liquor from the third crystallization was free 

 from potassium. This mother liquor was evaporated to small bulk, 

 neutralized with ammonia, and reduced and precipitated with hydrogen 

 sulphide. The filtrate after evaporation and expulsion of the ammo- 

 nium salts gave no spectroscopic flame test for potassium. 



The silver dichromate was not allowed to come in contact with water 

 or any solution except the 3 normal nitric acid solution. 



All of the above operations were carried out in platinum vessels. 



Sample II This sample was made exactly as in the case of Sample 

 I, except that chromic acid was employed instead of potassium dichro- 

 mate, and that both precipitation and crystallization took place from 

 0.8 normal nitric acid. The silver dichromate was crystallized five 

 times. 



Sample III. The most dilute nitric acid which was used in the 

 preparation of the silver dichromate was about 0.16 normal, solutions 

 of this concentration being employed in the precipitation and crystal- 

 lization of Sample III. This sample was made from chromic acid and 

 silver nitrate, and was six times crystallized from 0.16 normal nitric 

 acid. 



The chief difference in the purification of the three specimens, aside 

 from the concentration of acid used in their preparation, lies in the 

 fact that Sample I was prepared from recrystallized potassium dichro- 

 mate and Samples II and III from chromic acid. All three samples 

 were crystallized many times as silver dichromate. 



After the final drainage in the centrifugal apparatus, the crystals 

 were dried in an electric oven at 150° for several hours. Then they 

 were powdered gently in an agate mortar and kept in platinum vessels. 



The Determination of Silver in Silver Dichromate. 



In preparing the silver dichromate for analysis, the complete elimi- 

 nation of moisture by fusion of the salt was impossible, owing to the 



