RICHARDS. — ATOMIC WEIGHT OF BARIUM. 71 



An opalescent solution produced by the addition of hydrochloric 

 acid or argentic nitrate to a solution of argentic chloride often requires 

 many weeks to deposit its suspended salt, even when warmed and 

 shaken, if the Mask is kept wholly in the dark. The effort to evapo- 

 rate the solution by boiling in vessels partially closed to exclude the 

 dust, proved equally unsatisfactory. Hence in order to weigh very 

 small amounts of dissolved argentic chloride it was necessary to evap- 

 orate the solutions upon the steam bath in suitably protected glass 

 dishes. In order to determine the magnitude of any possible errors 

 which might arise from this method of treatment a number of check 

 experiments were made. In the first place dilute solutions of argentic 

 nitrate were evaporated to small bulk with the addition of nitric acid. 

 The evaporated solution was filtered through a small paper, and the 

 ash of the washed filter was dissolved in nitric acid and titrated with a 

 weak standard solution of amnionic sulphocyanate,* after the method 

 of Volhard. Three experiments showed that an amount of silver 

 equal to about 0.02, 0.01, and 0.00 milligram respectively had been 

 retained by the filter. These experiments proved that the precautions 

 taken to prevent dust or foreign 'materials from reaching the surface 

 of the solutions during the process of evaporation had been sufficient 

 for their purpose ; also, that no very important amount of silver was 

 held by the paper. 



Having determined these facts, the next step was to determine 

 whether all of a very small known amount of argentic chloride could 

 be recovered. A few preliminary experiments showed that in order 

 to reduce and recover all of the silver by the ignition of the filter 

 paper it was necessary to keep the precipitate in a finely divided state. 

 This end is easily accomplished by using a solution dilute enough to 

 prevent the clotting of the opalescent chloride. Thus one cubic centi- 

 meter of the weak standard hydrochloric acid precipitated enough 

 argentic chloride from a slight excess of the nitrate to require after 

 reduction by the burning of the filter 0.97 and 0.99 cubic centimeter 

 of the sulphocyanate solution. Although the evaporation was con- 

 ducted in dull diffused light, the argentic chloride was not discolored. f 

 These experiments showed that a small amount of argentic chloride 

 could be nearly all recovered from a small volume of solution. 



The next experiments were made with large volumes of solutions con- 

 taining about 0.2 gram of silver, a very little baric nitrate, and a small 



* See page 66 of this paper. 



t Compare Pohl, Jahresbericht 1851, p. 369. Quoted by Mulder, Silber- 

 probirmethode, p. 19. 



