DETAILS OF ANALYSIS. 



49 



Having thus, by many experiments, of which those given above 

 are merely samples, assured ourselves that the true end-point may be 

 decided by equality of opalescence when sufficient time is allowed to 

 complete the opalescent precipitation, it became an important matter 

 to make a conclusive series of analyses by this method. 



In the first place, a given piece of the purest silver was carefully 

 weighed and was dissolved in a flask upon the steam bath. For 

 every gram of silver about 3 milliliters of nitric acid, of density 

 about 1.20, were used. This density was secured by mixing pure 

 distilled nitric acid of the usual concentration with an equal volume 

 of water. After the silver was dissolved, more water was added and 

 the nitrous fumes were expelled by a brief, very gentle ebullition. A 

 tower of bulbs, ground to fit the neck of the flask, prevented any loss 

 of silver nitrate during the operations. A special experiment proved 

 the efficiency of the bulbsas a means of retaining spray. In this man- 

 ner of treatment nitric acid always remained in slight excess. 



Assuming a probable value for the atomic weight of sodium, 

 the exact weight in air of salt equivalent to the weight of the given 

 piece of silver was calculated and weighed out for fusion. There was 

 always a slight loss of weight when the salt was fused in the platinum 

 crucible, and if, after fusion, more than a milligram of salt was still 

 required, approximately the required amount of well-dried salt was 

 added at once by a platinum spatula. The amount so added never 

 exceeded a few milligrams and its introduction could have caused no 

 error, since in our laboratory salt was not hygroscopic. The exact 

 weight of salt corrected to vacuum was obtained. Any further still 

 slighter lack of equivalence was then made up by volumetric addition 

 of the proper quantity of the same solutions which were to bring the 

 mother liquor to equal opalescences after the precipitation. Of course 

 the recorded weights include all these small additions. 



Both salt and argentic nitrate were diluted to a concentration 

 about fifth normal, and were then mixed in complete darkness with 

 all possible precautions, the argentic nitrate being very slowly poured 

 into the salt solution. When in a day or two the precipitate had set- 

 tled and the mother liquor had become clear, the latter was examined 

 nephelometrically . 



It was found to be convenient, in determining the amount of a 

 deficiency of either silver or chloride, to calculate once for all the 

 extent of this deficiency in terms of the ratio of lengths of the two 

 nephelometer columns giving equal apparent opalescence. This was 

 in the first place calculated by means of the law of concentration effect ; 

 but practically a slightly different value, based upon experience, was 



