286 Day and Shepherd — Lime-Silica Series of Minerals. 



colder parts of the furnace. Unglazed porcelain offers no 

 protection against this iridium. The effect of this contamina- 

 tion is to make the thermoelements read too low. The error is 

 noticeable after a single hour's exposure (perhaps 1/2°), and 

 will gradually increase to fifty degrees or more in a few weeks 

 of continuous usage, the amount depending considerably upon 

 the temperatures employed. The remedy is simple and sure, — 

 use no iridium in the furnace. An element once contami- 

 nated in this way can only be restored by cutting off the exposed 

 portion. 



Above 1600° the platinum coil and the thermoelement gave 

 place to the iridium tube furnace and the Holborn-Kurlbaum 

 optical pyrometer. The adjoining diagram (fig 4) contains 



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sufficient details to show almost at a glance the operation of 

 the system. A straight iridium tube about 18 cm long and 

 4 cm in diameter is fed by an alternating current of low voltage, 

 led in through platinum and silver flanges at the ends. Fused 

 magnesia serves as insulating material and a base of magnesia 

 supports a small iridium crucible in the position indicated. A 

 small opening in the magnesia cover enables the pyrometer to 

 be sighted on any point within.* The furnace will reach tem- 

 peratures of 2100° C. and is almost indispensable at very high 

 temperatures where an oxidizing atmosphere is important. 



*This furnace was made for us by Dr. Heraeus of Hanau, Germany, to 

 whom, as well as to his associate, Dr. Kuch, we are indebted for many cour- 

 tesies. 



