1909] on Tantalum and its Industrial Applications. 597 



light, or by sending the same current through both lamps in series 

 and noting the difference in candle power. 



In conclusion, two interesting qualities of Ta should be noted — 



The first is that when a Ta filament is heated in a high vacuum 

 it will expel any oxygen that has combined with it. It is possible to 

 detect whether a filament contains any oxide by very gradually heating 

 it up, when the parts containing oxide will appear brighter than those 

 consisting of pure Ta, owing to the greater electrical resistance of the 

 oxide. 



These lamps have been purposely exposed to the air while they 

 were being exhausted and have become " spotty " in consequence, but 

 if they are raised a little above their proper voltage and left burning 

 for a few minutes, their filaments become quite uniform by the expul- 

 sion of the oxygen. 



The second is that Ta will act as a rectifier when used in an 

 electrolyte, that is to say, it will allow of the passage of the positive 

 current only in one direction. In the apparatus shown the positive 

 current passes through the lamp to a Ta anode, thence to a Pt cathode, 

 but in a very short time the Ta anode covers itself with a film of 

 oxide which stops the current. When the current is reversed the 

 lamp lights again and continues to burn. When an alternate current 

 is connected to the lamp it will also continue to burn, but with reduced 

 light. 



All these experiments are intended to show the remarkable quali- 

 ties of this material, and when they are fully appreciated and its 

 limitations are properly understood, there appears to be a great field 

 open to tantalum and its industrial applications. 



[A. S.] 



