286 



BURGESS AND SALE: PURITY OF PLATINUM WARE 



the comparison of columns 2 and 6, the former giving the stated 

 iridium content (and often accompanied by the statement that 

 there are no other impurities) and the latter giving the iridium 

 content as determined thermoelectrically by experiment and use 

 of figure 2. In several instances, notably for the crucible of 

 normal thermoelement platinum (a) of Heraeus and for Baker's 

 crucible (k) of 2.37 per cent iridium, the stated and found irid- 

 ium contents agree exactly. On the other hand there is a wide 

 divergence from t,he supposed and actual iridium content for 

 several of the crucibles. For example, / of the American Plati- 

 num Works, supposed to contain not over 0.50 to 1.5 per cent irid- 



Fig. 2. 



ium, actually contains an iridium equivalent of 2.50 per cent; and 

 even in the case of refined, specially refined and best crucible 

 ware, the equivalent iridium content is not inconsiderable. 

 Among the 23 crucibles examined there is but one, I of Baker 

 and Company, containing less iridium than stated. 



In comparing the stated contents, column 2 of the Table, it 

 should be born in mind that some of these crucibles, excepting 

 perhaps those marked refined or specially refined, may contain 

 iridium which is put or left in the crucible material purposely, 

 usually for the pm*pose of stiffening, or in the case of commercial 

 ware, because such Pt always carries Ir. The method here 



