MERCUROUS CHLORIDE BY HAROLD S. KING 119 



precipitate to the air and found that about half of the mercury 

 volatilized on standing, leaving a residue much lighter in color 

 than the original precipitate. This residue was identical with 

 the infusible white precipitate formed by the reaction between 

 mercuric chloride and aqueous ammonia. H. Saha and K. N. 

 Choudhuri" found that infusible white precipitate was soluble 

 in concentrated aqueous ammonia. When the precipitate 

 obtained by the action of ammonia on calomel was extracted 

 with this solvent, infusible white precipitate was found to go 

 into solution leaving a residue of metallic mercury. J. G. F. 

 Druce" found that the precipitate would not amalgamate 

 metallic copper, and at first announced that this was proof that 

 metallic mercury could not be present, but later he reversed his 

 opinion. 



It seemed to me that these proofs, especially the second, 

 were not rigid, because of the possibility of secondary reactions. 

 Therefore a purely physical proof was devised, which depended 

 on the difference in density between mercury and infusible white 

 precipitate. The freshly prepared precipitate was thoroughly 

 mixed with cold glycerine. The mixture was floated on a 

 layer of glycerine in a test tube and whizzed in a centrifuge. 

 The mercury settled out faster than the infusible white preci- 

 pitate so that the part that was thrown out last was very per- 

 ceptibly lighter in color, being nearly white. The part that 

 settled out first gave a residue of free mercury on treatment 

 with hydrochloric acid, while the white precipitate was dis- 

 solved without residue by the same reagent. The use of cold 

 glycerine as a suspending medium was necessary to prevent too 

 rapid deposition. There does not seem to be any possibility 

 for secondary reactions in this method, so it can be taken as 

 proved that the precipitate formed by the action of aqueous 

 ammonia on mercurous chloride is not a simple substance but a 

 mixture with metallic mercury as one of the components. 



The next point for examination is the mechanism for the 

 formation of the mercury. It has already been pointed out 

 that there is an equilibrium between mercurous chloride on the 

 one hand and metallic mercury plus mercuric chloride on the 



