38 ATOMIC WEIGHTS OF SODIUM AND CHLORINE. 



every particle of precipitate. In the most careful experiments the wash 

 bottle was provided with protecting bulbs to preserve the purity of the 

 water, or was dispensed with altogether, being replaced by a jet driven 

 by hydrostatic pressure. At this stage the cloth covering was neces- 

 sarily removed from the flask. The flask was finally rinsed with 

 ammonia, which was carefully tested for a possible trace of silver 

 chloride. 



On drying the precipitate the temperature was made to rise very 

 slowly above ioo during two or three hours. This deliberate drying 

 was found to be more effective than the immediate application of high 

 temperature, presumably because a hard coating was not then formed 

 about the mass of the precipitate. Finally the temperature was main- 

 tained at about 150 for two or three hours longer. In this way fairly 

 constant weight is easily attained, which fifteen hours' further heating 

 at 150 changes very slightly. The last traces of water were always 

 expelled by fusion, as is described below. 



In many of the experiments the silver chloride was dried in an 

 electric drying oven heated by hot platinum wires conveying a current 

 of an ampere or two.* When properly constructed this oven gives 

 very satisfactory results. The working of the oven is somewhat more 

 satisfactory if the platinum wires are not heated to redness. In some 

 of the later experiments the precipitate was dried in a clean copper 

 oven heated by a gas flame with no apparent difference in result. 



The precipitate thus collected and dried was carefully weighed 

 and then freed from adhering asbestos and transferred to a weighed 

 porcelain or quartz crucible for fusion. The crucible and contents 

 were tared against a similar crucible and contents, and the argentic 

 chloride under examination was then fused in a clean i oven constructed 

 from a large porcelain crucible, from the interior of which the products 

 of combustion of the gas were carefully deflected. The fusion is neces- 

 sary, because minute drops of water are always held by the hardened 

 argentic chloride in sealed cells. It is true that the loss on fusion is 

 not great, never amounting to 0.01 per cent of the weight of the pre- 

 cipitate, but it was enough to receive consideration. In every case it 

 was determined and the appropriate individual correction applied to 

 each weight. The maximum value of the correction was 0.008 per 

 cent and the minimum 0.003 P er cent. That the loss was not due to 

 volatilization of silver chloride is easily shown by the invariability of 

 the weight upon fusing a second time. That it was not due to accidental 

 reduction was shown in the later experiments by fusing again in a 



*Richards, Am. Chem. Journal, 22, 45, 1899. 



