382 J. C. Blcike— Colloidal Gold. 



results to be obtained with colloidal gold solutions would be 

 especially interesting in this connection, both because of the 

 great improbability of ordinary chemical union between the 

 gold and the basic radicals, and because of the ease and 

 thoroughness with which the precipitated gold could be 

 washed. 



Red colloidal solutions of gold prepared according to the 

 method just described were precipitated by solutions of 

 barium compounds, and the coagulum was analyzed for gold, 

 barium, and carbon, derived from the acetylene. Except 

 when otherwise stated, all the filtrations and weighings were 

 made on asbestos felt contained in a perforated platinum 

 crucible. The mixed coagulum, either washed or unwashed, 

 was ignited in a porcelain crucible and then treated with aqua 

 regia in small amount either at the ordinary temperature or on 

 the steam bath, the carbon being nearly all dissolved in the 

 latter case. After dilution, the undissolved carbon was filtered 

 off, washed, dried at 200°, weighed and ignited. The gold 

 was determined as the metal in the filtrate from the carbon by 

 precipitation with magnesium ribbon. The barium was pre- 

 cipitated as sulphate in the filtrate from the gold and weighed 

 after standing twelve hours on the steam bath. 



It was found that if the colloidal gold solution contained a 

 little unreduced gold chloride, the coagulum was coherent, 

 spongy, opaque, insoluble in water, and reflected brownish 

 yellow light. If, however, the gold chloride had been com- 

 pletely reduced, the coagulum was blue-black, non-coherent, 

 readily soluble in pure water to a blue colloidal solution, the 

 color being due to transmitted light. 



I. The Spongy Form of Gold. 



In the following experiments the gold chloride used in 

 preparing the colloidal gold solution was not quite all reduced, 

 and the colloidal gold, precipitated with a solution of a barium 

 salt, came down in the spongy form. The coagulum was 

 either washed in succession with the amounts of water given 

 in the table, the included liquid being finally pressed out of the 

 spongy precipitate as much as possible, or the coagulum was 

 not washed, but merely dried between filter papers. For pur- 

 poses of comparison the amounts of the barium compounds 

 (calculated as hydroxide) which were present in O'^""^^ of the 

 supernatant liquid are also given in the table. 



