EICHARDS. — ATOMIC WEIGHT OF STRONTIUM. 



385 



ing to Stas,* argentic bromide is wholly insoluble in water ; accord- 

 ing to Goodwin,! it is only very slightly soluble; while according to 

 Kohlrausch and Rose,f it is soluble to the extent of three tenths of 

 a milligram in a litre. The time during wliich chloride of silver is 

 shaken makes an enormous difference in the solubility, and it is not 

 impossible that a similar effect may occur here. Perhaps KoWrausch 

 and Rose did not agitate their precipitate so thoroughly as Stas did. 

 According to the present experience the purest silver bromide was 

 capable of yielding a filtrate which would give a very faint opalescence 

 with both silver and hydrobromic acid ; and this 

 effect usually diminished upon long continued agita- 

 tion. The method of determination used in this 

 series was based upon this fact. Somewhat less 

 silver than the amount required was added to the 

 strontic bromide, and a very weak standard solution 

 of argentic nitrate (the cubic centimeter contained 

 a milligram of silver) was dropped in until equiva- 

 lent solutions of silver and hydrobromic acid pro- 

 duced equal opalescence in two similar pipetted 

 portions of the supernatant liquid. Since the opa- 

 lescence was so faint that one could only with diffi- 

 culty see it at all under ordinary conditions, a piece 

 of apparatus, which may be named a " nephelome- 

 ter" (ve<ji4X7], a cloud), was devised for detecting it. 

 Two test tubes, holding each just thirty cubic cen- 

 timeters, were arranged in a wooden frame so that 

 two centimeters of the top of the tubes were in 

 darkness. The bottoms of the tubes were fitted 

 into the top of larger opaque tubes containing water, 

 and were provided with closely fitting cylindrical 

 shades, which could be raised or lowered independently over a gradu- 

 ated scale. All these contrivances prevented disturbing side reflections 

 from the «aeniscus at the top of the tube and the rounded glass at 

 the bottom. The two test tubes were slightly inclined towards one 

 another, so that the eye at a distance of eight inches could look directly 

 into both without change of position. Filled with pure water the 

 tubes appear absolutely black, even when exposed to a strong light ; 

 but an absurdly small amount of precipitate, which no ordinary means 



u 



JEm. 



Nephelometer. 



* Mem. de I'Acad. Belg., XLIII., Part II. Introduction. 



t Zeitschr. f. phys. Chem., XIII. 645. 



VOL. XXX. (n. S. XXII.) 25 



t Ibid., XII. 234. 



