OF ARTS AND SCIENCES. 61 



In this series, as in the last, the letters indicate different prepara- 

 tions. The two marked a and h were both made from washed chloride 

 of silver, dissolved in pure aqua-ammonia ; that marked c was made 

 fi-om pure nitrate of silver, first converted into ammonio-nitrate, with 

 the least possible excess of ammonia ; that marked d was precipitated 

 directly from a dilute aqueous solution of the same argentic nitrate, 

 without ammonia, and was therefore formed in an acid solution. They 

 were all precipitated with a supersaturated solution of hydric sulphide, 

 and during the precipitation and subsequent boiling a current of car- 

 bonic dioxide was passed through the liquid. After the material had 

 been placed in the nacelle for reduction, it was heated to 300'^, in a 

 current of carbonic dioxide, before the weight was taken. These facts 

 are stated, because, as will be seen, the close accordance of the results 

 obtained furnishes the strongest evidence of the uniform purity of the 

 material prepared in the several ways we have described, and gave us 

 great confidence in the perfection of our new method of precipitating 

 sulphides. 



Stas obtained for the atomic weight of sulphur when Ag = 107.93 

 the value 32.074, and the mean of our results differs from his by only 

 0.063. How small this difference really is, is shown by the fact that 

 even with the largest quantity of sulphide used, — which required a 

 platinum nacelle 5 inches long by 1|^ inches wide to hold the spongi- 

 form* mass of reduced silver, — the difference in question only corre- 

 sponds to 1-/^ milligrammes in the weight estimated ; and with the 

 smaller quantities — which required the largest porcelain nacelle we 

 could obtain — the difference only corresponds to about half a milli- 

 gramme. Still, the process is sufficiently accurate to show even this 

 difference ; for the extreme variations from the mean value in the 

 last series of results only corresponds for the larger quantities to -^ 

 of a milligramme, and for the smaller to -^^t^ of a milligramme of 

 the quantity estimated. The difference, therefore, small as it is, evi- 

 dently points to a constant error of some kind, which, as we suspect, is 

 caused by a slight volatilization of silver, even at this comparatively 

 low temperature, although we were unable to obtain any other evidence 

 of it. Hence, the following two additional determinations may be of 

 interest, in which the sulphide was reduced below a visible red heat, in 

 a small platinum nacelle, heated in a tube of hard glass : — 



* The production of moss silver in this process is a most beautiful phenome- 

 non, which has been described by Dr. Percy, " Metallurgy," I. 360, and more 

 recently by Professor Liversidge, of the University of Sydney. 



