22 ATOMIC WEIGHTS OF SODIUM AND CHLORINE. 



0.005 per cent of dissolved carbon. (Experiment 7 9, p. 61.) Of course, 

 however, a single experiment has but little exact weight in such a 

 case, except to show that the process is probably undesirable. 



Returning from these unsatisfactory methods to the one used very 

 early in his career by Stas, we tested once more the use of lime as a 

 support for the fusing silver. This seemed to be an especially appro- 

 priate substance for the containing vessel, because it is so easily 

 obtained pure , and because it is so hard to reduce that no metallic calcium 

 could be dissolved by silver. But, of course, the silver must be satu- 

 rated with dissolved lime, and it remained to be proved that a saturated 

 solution of lime in fused silver is so dilute as to introduce only a 

 wholly insignificant amount of impurity. This was easily shown by 

 actual experiment. Ten grams of silver fused on lime were dissolved 

 in pure dilute nitric acid in a platinum dish and precipitated as sulphide 

 by pure hydrogen sulphide gas, led in through a platinum tube. After 

 filtration on a platinum funnel, the remaining liquid was evaporated 

 to dryness in platinum. The minute trace of residue, probably due 

 to traces of dust, on moistening with hydrochloric acid gave no trace 

 of the calcium bands in a spectrometer which showed them plainly 

 from a solution containing 0.01 milligram of lime in 1 cubic centimeter. 

 Hence it is evident that less than this amount of lime was dissolved 

 by the silver, i. e., less than 0.0001 per cent, if any. 



This experiment settled the question concerning the supporting 

 substance, but of course the lime must be perfectly pure. If the lime 

 is partially made from the nitrate, it must be very thoroughly ignited 

 before it is used, otherwise the silver may dissolve some of the oxygen 

 caused by the decomposition of the salt. This was shown by the 

 careful testing of a quantity of silver fused on a new boat, which had 

 not been thoroughly ignited. (Exps. 72 to 76, p. 61.) A boat in 

 which pure silver has already been fused has an advantage in this 

 respect, as well as because every accessible trace of reducible material 

 must have been dissolved out of it. Of course the lime used in the 

 present experiments was especially purified, as will be related. 



It is true that even when carefully prepared a boat of lime is likely 

 to be so friable as occasionally to cause a partial coating of the button 

 of silver with the powdered lime, but the great surface tension of 

 molten silver keeps this all on the surface, whence it is easily dis- 

 solved by means of the dilute nitric acid always used by us to clean 

 the metal. Scott, who has objected to the use of lime, again on the 

 basis of a single experiment, does not mention having taken this 

 necessary precaution. 



