70 PROCEEDINGS OP THE AMERICAN ACADEMY. 



nous argentic chloride was soluble to a very large extent (over ten 

 milligrams to a litre) in the purest water, and that nitric acid or the 

 alkaline nitrates had no appreciable effect upon the solubility. The 

 pulverulent precipitate obtained by continual shaking he found to be 

 much less soluble, ouly 0.7 milligram being dissolved by a litre of 

 pure water. Nitric acid increases to a sensible degree this last quan- 

 tity, the increase being approximately proportional to the amount of 

 acid present. 



Mulder knew well that it was possible wholly to precipitate either 

 silver or chlorine by the addition of a sufficient amount of the other 

 reagent, but wrongly assumed that the amount of the reagent required 

 was a direct measure of the solubility of argentic chloride. This mis- 

 conception probably arose from a theoretical discussion of very doubt- 

 ful value; its effect was to make Mulder's estimate of the solubility 

 rather excessive. Stas recognized the fact for the first time, that much 

 more silver is required to precipitate the chlorine in solution as silver 

 chloride than is enough directly to combine with it. A very tedious 

 series of experiments led him to the conclusion that for every mole- 

 cule of argentic chloride dissolved just three molecules of argentic 

 nitrate or of a chloride were required to complete the precipitation. 

 In 1881 Cooke called attention to some of the same facts.* 



Much time has been spent during the present research upon the 

 subject, for its importance with regard to the analysis of baric chlo- 

 ride was very evident. The experiments were largely a repetition of 

 Stas's, with occasional variations suggested by the case in hand. 

 Mulder and Stas having wholly done away with the old idea of the 

 insignificance of the action of light on the haloid compounds of silver, 

 the greatest care was taken with regard to the protection of the pre- 

 cipitates from diffused light. Work with them was performed wholly 

 in a dark room lighted by means of illuminating gas shining through 

 thick orange glass ; and even here the flasks were always covered by 

 two or three thicknesses of black cloth when left to themselves. 



In determining the solubility of argentic chloride a number of ex- 

 periments were made, first by celorimetiic and " opalimetric " methods. 

 The coloration produced by hydric sulphide and the opalescence pro- 

 duced by hydrochloric acid were compared with similar phenomena 

 produced by known amounts of silver. Very much experience is 

 needed to obtain satisfactory results with either of these methods, so 

 that a more direct method of determination was sought. 



* These Proceedings, XVII. 7. Cooke also studied the action of hot water 

 upon argentic chloride. 



