190 PROCEEDINGS OF THE AMERICAN ACADEMY. 



other end was sealed to a small hard glass tube which was surrounded 

 with a damp cloth during the fusion of the salt in order to facilitate 

 condensation of any silver or arsenic compounds vaporized from the 

 salt. As a matter of fact, very little sublimation actually took place. 



In order to fuse silver arsenate within the hard glass tube it was 

 necessary to use the hottest flame of the blast lamp, the tube being 

 covered with a semi-cylinder of sheet iron. Furthermore, since at this 

 temperature even the hard glass became very soft, it was found neces- 

 sary to wrap the tube with asbestos and closely wound iron wire for 

 several inches at the point where fusion took place. This also served 

 to distribute the heat more evenly and to prevent the tube from crack- 

 ing during the experiment. 



Just before the salt was fused a carefully weighed U-tube containing 

 resublimed phosphorus pentoxide was attached to the end of the tube, 

 and beyond this was joined another similar tube which served as a pro- 

 tection against any moisture which might diffuse back into the weighed 

 tube from the outside air. These phosphorus pentoxide tubes were 

 provided with one way ground glass stopcocks lubricated with Ram- 

 say desiccator grease. 



The salt was heated for twenty-five minutes with the hottest flame 

 of the blast lamp, being then completely fused, and was further heated 

 for thirty-five minutes at a considerably lower temperature in order to 

 make certain that all moisture was carried into the absorption tube by 

 the current of air. Finally the phosphorus pentoxide tube was reweighed. 



The pentoxide tube was weighed by substitution with the use of a 

 counterpoise of the same size and weight filled with glass beads. Be- 

 fore being weighed both tubes were carefully wiped with a damp cloth 

 and were allowed to stand near the balance case for one hour. One 

 stopcock in each tube was opened immediately before the tube was hung 

 upon the balance, in order to insure equilibrium between the internal 

 and external air pressure. The stopcock of the counterpoise was left 

 open during the weighing. Owing to the considerable length of time 

 required for the tubes to come to equilibrium on the balance, it was 

 considered unsafe to leave the stopcock of the pentoxide tube open dur- 

 ing the weighing. As a check on the first weight of the pentoxide tube 

 one stopcock was opened and closed and its weight determined a second 

 time. Ordinarily no change in weight was observed. 



Since it seemed possible that the hard glass tube itself, when heated 

 nearly to fusion, might give off traces of water vapor, two blank deter- 

 minations were made by heating the empty hard glass tube in exactly 

 the same way as in the water determinations. These determinations 

 showed a gain in weight of the pentoxide tube of 0.00022 and 0.00037 



