EQUALITY OF OPALESCENCE. 



47 



Even when all these precautions had been taken, however, incom- 

 prehensible irregularities were still present in the results. It was then 

 found that the time allowed for the formation of the opalescence had 

 not been adequate ; that in some cases one tube had clouded much 

 faster than the other, but that in all cases neither tube had attained 

 maximum intensity. Often as much time as eight hours was needed 

 for the attainment of this maximum ; but the time must not be too 

 greatly extended, or some of the precipitate will be deposited on the 

 bottom of the tube. The performance of the precipitation when 

 chloride is in excess is thus not wholly parallel with that when silver 

 is in excess, already discussed on p. 28. 



In view of these facts, a new comparison was made. A quantity 

 of silver chloride was precipitated in red light from cold decinormal 

 solutions of silver nitrate and sodic chloride, washed twelve times with 

 water weakly acidified with nitric acid and eight times further with pure 

 water, the last washings being made in absolute darkness. Pure water 

 containing such quantities of nitric acid and sodic nitrate as would 

 occur in an actual mother liquor from a quantitative precipitation, 

 was then added, and the mixture was allowed to stand with occasional 

 agitations for a day. In making the nephelometer observations on 

 the decanted liquid, sufficient time was allowed for the precipitates to 

 become constant. Three completely separate trials were made in the 

 nephelometer ; and of these are recorded below the depths of liquids 

 which seemed to be equally opalescent. 



These observations, made with great care, settled every doubt 

 about the correctness of the equal-opalescence end-point. Thus it is 

 clear that Stas encountered difficulties with this method only because 

 he, like all the previous experimenters, did not wait long enough to 

 allow the opalescence really to establish itself in the absence of elec- 

 trolyte to hasten precipitation. 



In the course of these experiments several subordinate points 

 were noted which are worth recording. When the opalescent precipi- 

 tates were made directly in the tubes from sodic chloride and argentic 



