RICHARDS AND ARCHIBALD. — ATOMIC WEIGHT OF CAESIUM. 453 



and all the other small causes of error. The most serious cause of 

 difficulty is the dilemma introduced by two conflicting tendencies of 

 argentic chloride, — namely, its solubility and its tendency to absorb 

 argentic nitrate. When it is washed with very dilute argentic nitrate 

 it does not dissolve, but it retains some of the dissolved nitrate ; on the 

 other hand, when repeatedly shaken with pure water it retains no im- 

 purity, but dissolves to an appreciable extent. In analyses 1 to 10 and 

 14 to 22, below, the chief precipitate was thoroughly washed by shaking 

 with a solution containing a very small known amount of ionized silver, 

 and the residual liquid moistening the Gooch filter was displaced by 

 a few cubic centimetres of pure water. This reduces each error to a 

 uiinimum, and almost balances their slight residual effect; but the fused 

 argentic chloride thus obtained is not perfectly white and pearly. There- 

 fore, in the final determinations (11 to 13, and 23 to 25) a safer but 

 more troublesome procedure was adopted. After the precipitate had 

 been well shaken in its glass-stoppered Erlenmeyer flask with several 

 portions of wash water, containing a known amount of silver nitrate, 

 and rinsed with about 50 c.c. of pure water, it was again shaken vio- 

 lently with five separate portions of water (in all about 500 c.c.) which 

 contained no silver nitrate. This treatment was found to remove all 

 traces of adsorbed silver nitrate. These wash waters were kept by 

 themselves, and the amount of dissolved silver chloride which they 

 contained was estimated by means of the nephelometer, this correction 

 being added to the observed weight of silver chloride. Argentic 

 chloride thus washed fuses into a beautiful translucent pearly white 

 mass. 



The balance used in this research was the one which has been used 

 in the inveptigations of many other atomic weights.* It is short armed, 

 and sensitive to about one-fiftieth of a milligram, with the largest load 

 used in this work. The weights were of brass, gold plated. They 

 were very carefully standardized, according to the method suggested 

 by one of us,t and were used in no other work. 



All weighings were reduced to the vacuum standard. For this pur- 

 pose the specific gravity of caesic chloride was determined. 1.013G and 

 0.8877 grams of the salt were found to displace, respectively, 0.2313 

 and 0.1965 gram of benzol at 20°. The specific gravity of the benzol 

 referred to water at 4° was 0.880, hence the two values for caesic chlo- 



* Richards, Proc. Am. Acad., 26, 242 (1891). 



t Richards, Jour. Am. Cliem. Soc, 22, Ml; Z. Phys. Cheni., 33, 005 (1900). 



