RICHARDS AND ARCHIBALD. ATOMIC WEIGHT OF CAESIUM. 461 



in this Laboratory in other work. Its general character will be seen 

 from the diagram taken from the original paper. * 



A large especially made crucible of Berlin porcelain fits tightly into a 

 jacket or furnace of fire clay ; the platinum crucible to be ignited is 

 placed inside this, being protected from the porcelain by a spiral of 

 platinum wire between the two crucibles. As a further protection from 

 the impurities of the gas flame, the platinum crucible is fitted with a 

 small porcelain cover, while a larger cover is placed above it. A circular 

 opening is cut in two ignited asbestos sheets, lying on top of the furnace, 

 into which the larger outside crucible fits tightly ; these sheets serve as a 

 further means of deflecting the impurities from the flame, since the only 

 escape for the products of combustion is through the draught holes in the 

 side. The asbestos sheets are not shown in the diagram. By means of 

 this furnace the crucible can be heated at a uniform temperature tor any 

 length of time. 



Experiments were made to ascertain if in this furnace any platinum 

 volatilized from the crucible at high temperatures. When half full of 

 silicic dioxide, no appreciable loss or gain was observed, either iu the 

 Bunsen flame or in the full flame of the blast lamp. When empty, how- 

 ever, although the crucible lost nothing in the Bunsen flame, it lost 

 0.00012 grams when heated for an hour in the blast. Evidently traces 

 of platinum were volatilized from the inside of the vessel into the open 

 air space. The outside annular space was probably kept saturated with 

 platinum vapor from the platinum wire. Since the crucible was always 

 more than half full of silica when placed in the furnace during the 

 work described below, it was assumed that no platinum was lost by 

 volatilization. 



When the precautions indicated in this chapter are carefully followed 

 no further complications arise, and the method easily yields constant 

 results which seem to be unimpeachable. 



The chief objections to the method lie in the fact that the nitrate must 

 be wholly free from water (an end easily attained by gentle fusion in 

 the present case f), and the further fact that any error which may occur 

 is intensified in the calculation, as is always the case when the two sub- 

 stances weighed both contain either the element whose atomic weight is 

 soujiht or the standard of reference. 



* Richards, Proc. Amer. Acad., 33, 399 (1898); Am. Chem. J., 20, 701 (189G). 

 t Stas (Aronstcin), Untersuclmngen, 2S5 (18G7). 



