191 



After a third fusion, removal of the product, and heating over the 

 Meeker burner the crucible weighed 



25.0097 25.0098 

 After removal of the ferric oxide and reheating, 

 25.0080 25.0080 25.0080 25.0080 

 Amount of ferric oxide by titration, 0.0063 gram. 



After a fourth fusion, removal of fusion product, successive heat- 

 ings gave 



25.0103 25.0100 25.0100 

 After removal of the iron oxide, 



25.0085 25.0085 

 Total amount of ferric oxide by titration, 0.0051. 



It seems that in fusing the clay and sodium carbonate mixture a 

 very small amount the ferric oxide formed, or the ferrous oxide present 

 is reduced, the iron dissolving in the platinum. When the crucible is 

 afterwards heated to a high temperature, the ii-on is driven to the sur- 

 face and reoxidized, thereby becoming soluble in the acid. 



It was thought that this might be prevented by adding a small 

 amount of potassium nitrate to the fusion mixture before making the 

 fusion. A few experiments were made to test this hypothesis. 



Weight of the crucible before fusion, 25.0079. 



To the mixture of 0.5 gram clay and 2.5 grams sodium carbonate 

 was added 0.3 gram pure potassium nitrate, the fusion made and the 

 product removed in the usual way. The crucible was then heated eight 

 times, fifteen minutes each, and weighed after each heating, the ferric 

 oxide being removed before the succeeding heating. The weights were 

 as follows: 



25.0104 25.0103 25.0103 25.0103 25.0091 25.0087 25.0079 25.0079 

 The total amount of ferric oxide obtained by titration, 0.0021 gram. 



A second fusion with the addition of 0.5 gram of potassium nitrate 

 brought the weights of the crucible to the following: 



25.0125 25.0122 25.0121 25.0120 25.0120 25.0120 

 The total amount of ferric oxide by titration, 0.0015 gram. 



