ARSENIC. 345 



8. Dry mercury compounds, when mixed with sodium carbonate 

 and potassium cyanide, and heated in a narrow test-tube, are decom- 

 posed with liberation of metallic mercury, which condenses in small 

 globules in the cooler part of the tube. 



9. A piece of bright metallic copper when placed in a slightly acid 

 mercury solution becomes coated with a dark film of metallic mer- 

 cury, which by rubbing becomes bright and shining, and may be 

 volatilized by heat. (See Solution tension, page 319.) 



10. All compounds of mercury are completely volatilized by heat, 

 either with or without decomposition. 



Tests 2, 4, 7, 8, and 9 will show the presence of mercury in any of 

 its compounds. Those that are insoluble in water may be dissolved 

 by a little concentrated hydrochloric with a few drops of nitric acid, 

 forming mercuric chloride. Excess of acid is removed by evaporation . 



Antidotes. Albumen (white of egg), of which, however, not too much should 

 be given at one time, lest the precipitate formed by the mercuric salt and 

 albumin be redissolved. The antidote should be followed by an emetic to 

 remove the albuminous mercury compound. 



Ionic conditions. The simple mercury compounds give mercurous ions, Hg", 

 and mercuric ions, Hg", which show different behaviors toward reagents, as 

 seen in the tests above. The mercury ions are colorless, and are not formed 

 extensively from any compound. The disinfecting and poisonous properties of 

 mercury compounds depend upon the presence of the ions. Mercuric chloride 

 in the dry state is inactive, and its solution in alcohol or ether is almost inert 

 as a disinfectant, because there are practically no mercury ions formed. 



Salts in general have a high degree of ionization, but salts of mercury and 

 cadmium are remarkable exceptions. For this reason mercury salts show some 

 peculiar behaviors. For example, the halogen salts of mercury dissociate so 

 little (the bromide and iodide less than the chloride) that they are scarcely 

 affected by sulphuric or nitric acid. Sodium chloride with sulphuric acid gives 

 hydrochloric acid, and with nitric acid it gives chlorine. Mercuric cyanide, 

 Hg(CN) 2 , is so minutely dissociated, that the presence of either Hg" ions or 

 (CN)' ions cannot be shown by precipitation with many reagents. Thus, silver 

 nitrate does not precipitate the (ON) 7 ions, as AgCN, nor do alkalies precipitate 

 Hg* * ions, as HgO. But hydrogen sulphide precipitates mercury from any of 

 its soluble compounds, because mercuric sulphide is practically completely in- 

 soluble and unionized. It is for this reason that the sulphide is not dissolved 

 by any acid, even when concentrated and heated, except nitrohydrochloric. 



Mercury is deposited from all its compounds, whether soluble in water or 

 not, by the metals higher up in the electrochemical series. Hence, it is not 

 advisable to use vermilion in paint to be applied to metallic surfaces. Eed lead 

 is better for this purpose. 



The complex salts which mercuric chloride forms with alkali chlorides dis- 

 sociate in part so that mercury is contained in the complex negative ion, and 

 to this extent loses its disinfectant property. The ionization equation for this 



