When the action of nitric acid is over, the solution must be at once separated from the excess of mercury to prevent the formation of basic salts. If the salt has been properly made, it is completely soluble in water, but if a lemon yellow precipitate is formed on dissolving, the nitrate contains a basic salt, which can only be dissolved by warming and adding more nitric acid. =Mercuric Nitrate=, Hg(NO₃)₂, is most simply obtained by warming mercury with very strong nitric acid. The heating must be continued until a test portion of the solution no longer gives a precipitate with hydrochloric acid. When this solution is evaporated, nitric acid is given off, and a salt crystallising in white needles is obtained, which dissolves in water with the separation of a yellow basic salt. It is therefore better to use the hot solution, which contains a little free acid, without evaporating. Instead of mercurous and mercuric nitrates, the corresponding sulphates may be used, but the chlorides are more frequently employed since they can be readily obtained from the makers. =Mercurous Chloride= (Calomel), HgCl, is obtained pure by adding common salt solution to a solution of mercurous nitrate and washing the precipitate, which is insoluble in water. =Mercuric Chloride= (Corrosive Sublimate), HgCl₂ a common article of commerce, is prepared by heating a carefully made mixture of mercuric sulphate and common salt, when mercuric chloride sublimes. It is a white crystalline mass, soluble in 13·5 parts of water at 20° C., and soluble in 3 parts of alcohol. Although all mercury compounds are very poisonous, corrosive sublimate requires particular care in handling, since its easy solubility makes it surpass all other mercury compounds in poisonousness. The mercuric sulphate required in the above preparation is obtained by heating mercury with sulphuric acid. Corrosive sublimate can also be prepared by adding hydrochloric acid to mercurous nitrate, and heating, with gradual addition of hydrochloric acid until a clear solution is formed, from which mercuric chloride crystallises on cooling. =Silver Compounds.=—Silver nitrate, AgNO₃, is the only one of importance here. It is obtained by dissolving silver in nitric acid, when a blue solution is obtained because commercial silver contains copper, evaporating the solution to dryness, melting the residue, and keeping it molten until all the copper nitrate is decomposed. This point is recognised when a small portion of the melt dissolved in water does not give a blue colouration with excess of ammonia. Fused silver nitrate forms a white crystalline mass readily soluble in water and turning black when exposed to light, like many other silver compounds.