436 



EEPOET 1881. 



The platinum and silver are thus very nearly in the prescribed pro- 

 portion, and the amount of impurity is inconsidei'able. An assay of a 

 piece cut from one end of the wire 1'578 m.m. in diameter, gave the pro- 

 portion of platinum as 31"34 %. 



A calibration made from inch to inch in its length showed the 

 conducting power to be sensibly uniform. 



Bar B. 



The temperature-coefficient of this bar was 0'0308%. The specific 

 gravity and absolute resistance were abnormal, aiid a calibration showed 

 it was quite irregular in conducting power, the casting being porous and 

 full of holes. 



Messrs. Johnson and Matthey failed in their attempt to roll or draw 

 it down into wire, but a small portion was remelted and drawn into wire 

 0'67 m.m. in diameter, of which the specific gravity was 13-354, the 

 temperature-coefficient "0282%, and the calculated resistance of 1 metre 

 of the wire, 1 millimetre in diameter, 0"238 ohms. It was obvious, 

 therefore, that even if the alloy was of the desired composition, on the 

 average, it had such a want of uniformity that an analysis was not worth 

 making. 



Bar {dental alloTj). 



The following Table gives the observations made with this sample 

 of alloy from the bar stage to that of very fine wire. 



Messrs. Johnson and Matthey's analysis of this alloy was as 

 follows : — 



Platinum , . 28-95 



Silver 70-50 



Rhodium 001 



Iridium 0-02 



Iron 002 



Lead, with minute traces of copper and loss . . . 050 



100-00 



An assay of one end of the wire of the diameter 1-625 m.m. gave 

 28'44 % as the proportion of platinum ; but it would appear, from the 

 third observation of specific gravity in the above table, that the com- 

 position could not be uniform — indeed the bar was calibrated in six 

 parts, both at the high and low temperature, and the temperature- 



