388 



of' the higfi melting- point of arsenic. Owing fo the (echnioal dif- 

 ficulties of making such an indicator, its sensibility was sonje- 

 what inferior to that of the indicator made of glass, still some of 

 the indicators which were nsed, could keep their sensibilities within 

 one centimeter of mercury, being sufficient for the present purpose. 



The Material. 



Merck's metallic arsenic was used after several purifications. At 

 first the finely powdered sample was subjected to repeated subli- 

 mation in a vacuum by the aid of a large Heraeus electric fuiiiace, 

 the temperature of the furnace was maintained at a little over 

 500° C. in the first sublimation and at nearly 600° C. in the final 

 one. The gray modification thus prepared was again very finely 

 powdered, and was extracted by carbon disnlphide in a Soxlet appa- 

 ratus for 24 hours. A small quantity of arsejiic oxide, which would 

 still remain in the above purified sample, must be reduced by 

 hydrogen current in the pressure indicator itself. 



The Filling of the Sample in the Pressure Indicator. 



About 10 gr. of the sample was placed into the bulb of a quartz 

 indicator, and a hard glass capillary tube was introduced into the 

 bulb of the indicator, so that the end of the capillary tube was just 

 in the spring of the indicator. Then the indicator was heated from 

 outside by Bunsen burners at 500° C; during the heating of the 

 indicator a current of purified hydrogen was passed into the bulb 

 and its spring tlirough the above mentioned capillary tube, so that 

 a small quantity of arsenic oxide, which still remained in the sample, 

 was at first sublimated and the rest of it reduced to pure arsenic. 

 After a sufficient sublimation in this way, the remainder of the 

 sample in the bulb became perfectly pure brilliant metallic arsenic. 

 Then the indicator was completely evacuated and the bulb of it 

 was sealed up. It was always observed that if arsenic was sublimated 

 in a vacuum, even at room temperature, at first it appeared as the 

 yellow modification, which would be soon transformed into the black 

 modification. 



The Furnace of the Pressure Measurement. 



A special furnace was constructed for the purpose of keeping the 

 indicator at constant temperature, even at very high temperature. 

 A large iron block of 14 cm. in diameter and of 30 ccm. in height 

 was heated electrically by nichroin wire. In the middle of this iron 

 I'lock, a hole of 3 cm. in diameter and of 25 cm. in deptli was 



