192 



A third fusion, using again 0.5 gram of potassium nitrate resulted 

 in tlie following weights: 



25.0154 25.0152 25.0149 25.0149 25.0149 25.0149 



No ferric oxide was detected by titration although a trace of ferric 

 oxide was observed in the crucible. 



A fourth trial with 0.5 gram of the nitrate resulted as follows: 

 25.0182 25.0181 25.0181 25.0178 25.0170 25.0165 25.0165 

 Total amount of ferric oxide by titration, 0.0025 gram. 



It will be seen that the amount of iron absorbed by the crucible 

 is sufficient to be taken into account in making an accurate analysis. 

 In other words, after making a clay fusion, the crucible should be 

 heated to a high temperature and the ferric oxide formed dissolved 

 out and added to the vessel containing the main fusion product. Fur- 

 thermore, it is seen that treatment with potassium nitrate is not a 

 satisfactory way of avoiding the trouble, for while it does prevent the 

 absorption of the iron to a large degree, it is the means of introducing 

 other foreign substances into the crucible which may prove undesirable. 



That this absorption of iron is not a peculiarity of this particular 

 crucible, due to the presence of some other metal alloyed with the 

 platinum, would seem to be indicated by the fact that the same phe- 

 nomenon was observed in connection with two other crucibles purchased 

 at different times and from different dealers; that it was not due to 

 some unusual property of this particular clay is evidenced by the fact 

 that the same thing occurred with clays obtained from widely different 

 sections of the State. 



A further study of this behavior is in progress. 



Since the above paper was submitted for publication, the chief cause 

 of the phenomena described has been discovered. The crucibles in which 

 the fusions were made were heated over Meeker burners. In order that 

 they might be heated to the highest temperature obtainable from these 

 burners the crucibles were supported just above the top of the burners. 

 As a result they were more or less enveloped in an atmosphere of re- 

 ducing gases and it was due to these gases rather than to the organic 

 matter in the clay that the iron was brought to a condition to be ab- 

 sorbed by the platinum. When these fusions are made with a good blast 



