BURGESS AND SALE: QUALITY OF PLATINUM WARE 



379 



scale of ordinates the iridium and rhodium contents of platinum 

 corresponding to these EMF's. It is seen that for practically 

 pure platinum the loss in weight is about 1 mg. per hour per 

 100 cm- at 1200° (crucibles a, b, c, d, e). For platinum contain- 

 ing 7.5 per cent rhodium (crucibles m and n) the loss is about 

 0.5 mg. per hour, while 2 

 per cent iridium in plati- 

 num (crucible i) increases 

 this loss to 2.5 mg. As 

 shown, the heating losses 

 may be taken as approxi- 

 mately proportional to the 

 iridium or rhodium con- 

 tent for crucibles that are 

 practically free from iron. 

 Iron appears to lessen 

 somewhat the loss of 

 weight on heating (see 



crucibles g, h, j, k, 1, of Lc,b m «. IMb) per lOO c«2 per e hr. he^t. 



Fig. 1) but its presence is Fig. i 



objectionable on account 



of the soluble oxide formed on the crucible surface. The chem- 

 ical analysis and magnetic measurements place the crucibles in 

 only approximately the same order as to iron content; the 

 magnetic susceptibility is not, however, proportional to the iron 

 content. 



A microscopic examination of a crucible that has been heated 

 will oftentimes aid in deciding whether it contains rhodium or 

 iridium as the principle impurity, the latter usually showing 

 heavier crystal boundaries and the former imparting a char- 

 acteristic skew structure. 



It appears, therefore, to be possible, from thermoelectric and 

 microscopic exaixiinations of a crucible, to predict its probable 

 loss of weight on heating within limits close enough for analytical 

 purposes. 



Whether crucibles have been long in use or not, after the first 

 two or three heatings and acid washings, appears to make little 



