RICHARDS AND ARCHIBALD. — ATOMIC WEIGHT OP CAESIUM. 459 



was tried, but its use was abandoned because of the very great difficulty 

 found in expelling every trace of water from it. All the experiments 

 described below were made with the pure powdered quartz sand. 



A rough experiment was first made in order that one might see where 

 errors were likely to appear and what special precautions would have to 

 be taken. 



A large platinum crucible containing a short platinum spatula was half 

 filled with the silicic dioxide, ignited for a short time, cooled in a desic- 

 cator and weighed, using another platinum crucible containing an equal 

 amount of sand as a tare. Some pure powdered potassic nitrate was 

 then added to the silicic dioxide, and the two were thoroughly mixed 

 with the platinum spatula. The whole was again weighed in order to 

 find the weight of nitrate added. Subsequently a large amount of the 

 dioxide was spread over the top until the crucible was three-quarters 

 full, and the whole was weighed again. This layer on top serves to 

 catch any of the nitrate which might otherwise be carried away with the 

 escaping vapors of the oxides of nitrogen. In order to find whether or 

 not any moisture had been taken up during the mixing, the crucible was 

 heated in the electric oven for two hours at about 150° C, cooled in a 

 desiccator and weighed. The loss actually found after this additional 

 drying never amounted to more than 0.00004 of a gram. The last 

 weight was of course taken as the true one. 



The mixture was now very carefully heated in the flame of the Bunsen 

 burner, keeping the temperature at a comparatively low point for several 

 hours and gradually increasing the heat until the temperature of the 

 blast lamp was reached. 



The melting point of potassic nitrate is about 339° C, while the caesic 

 nitrate melts near 414° C* ; the full heat of a Bunsen burner should 

 bring the silicate to a constant weight in either case. 



It was soon noticed that the nitrate had a tendency to creep over the 

 sides of the crucible, when a little above the fusing point. That part of 

 the nitrate next the platinum, not being entirely surrounded by the sand, 

 evidently had not the same opportunity of combining with the silica as 

 the remainder of the salt ; the exposure of this nitrate obviously might 

 cause a loss by sublimation at a high temperature. 



This danger was eliminated by using a small platinum crucible inside 



* This temperature was found in a capillary hard-glass tube by means of a 

 nitrogen-filled mercury thermometer, using a bath of potassic nitrate. The ther- 

 mometer was suitably standardized and corrected for the cool column. 



