10 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



1 



vapor due to diffusion from the mercury in the pump or gauge could 

 never exceed the vapor pressure of mercury at the temperature of the 

 liquid air. The tube G, when surrounded with the liquid air, was suf- 

 ficient safeguard against the entrance of water vapor with the gas. 



The method of removing all water vapor and mercury vapor already 

 in the apparatus beyond the tube E was that of repeated exhaustion 

 ^ and filling with the gas to be exper- 



imented with, the whole apparatus 

 ^ meanwhile being kept at a high 

 temperature. 4 



At the first exhaustion, when the 

 pressure had been reduced to a 

 few centimeters of mercury, the tube 

 G was surrounded by the electric 

 heater, and the heat was applied 

 to the oven in which the viscosity 

 apparatus is placed. Practically the 

 whole apparatus, except the gas 

 generator, was kept hot while the 

 pumping proceeded. After a fair 

 vacuum was reached the pump was 

 stopped and the hydrogen from the 

 generator was allowed to enter very 

 slowly, passing first over phosphoric 

 pentoxide, and then over spongy 

 platinum, heated in a combustion 

 tube, before entering the tube G. This filling process was followed 

 by another exhaustion under the same conditions. After the appara- 

 tus had been exhausted and filled a number of times in this way, 

 when it seemed certain that the apparatus and the pores of the char- 

 coal were filled w T ith fairly pure hydrogen, the heater was removed 

 from G, and the vessel containing the liquid air substituted for it. 

 The same process of alternately exhausting and filling was continued, 

 great care being taken in filling to allow the hydrogen to pass very 

 slowly so that the drying process might be complete. Keeping the 

 apparatus at a temperature of about 150° C. served to promote the evap- 

 oration of the mercury, which in all probability adhered to the inner 

 glass surfaces. Comparatively large quantities of pure dry hydrogen 

 were allowed to pass into the vessel and were then taken out. Each 

 exhaustion would assuredly sweep out some vapor if it was present. 



FIG. 2 



The suspended disk was, of course, lowered before this operation began. 



