MERCURIC SALTS CHANGE INTO EACH OTHER. 1(37 



free from mercuric salt, because it cannot be asserted that a very little, 

 if present, would not have become again mercurous in contact with 

 mercury in the act of dissolving in dilute nitric acid. But it is certain- 

 ly a mistake to hold, with him, that mercuric oxide, as the weaker 

 base, is to be found in the precipitate and not in the mother-liquor. 

 The precipitate can, indeed, be easily rendered mercuric by pouring off 

 its mother-liquor and heating it several times with fresh water, a fact 

 pointed out by Kane (1838). It then becomes more and more basic 

 by loss of acid and highly mercuric, no doubt, now by dissociation. 



Dissociation of mercurous nitrate by heat. — I do not wish to affirm 

 that the dissociation of mercurous nitrate does not go on slowly at the 

 common temperature in the absence of light ; it may do so, but so 

 slightly as to escape detection. It does not follow 7 that it does do so, 

 because of its dissociation at temperatures not much above the common, 

 for, while hot mercury oxidises, there is conclusive evidence that 

 cold mercury does not (Barfoed). The experiments now to be de- 

 scribed exhibit dissociation occurring at slightly elevated temperatures 

 in the dark. 



About one gram of normal hydrated mercurous nitrate, free both 

 from metallic mercury and from mercuric nitrate, was put with 10 cc. 

 of water in an Erlenmeyer flask, in order that its solution might 

 present a relatively large surface for evaporation. A reflux condenser 

 was connected with the flask to prevent loss by this evaporation and a 

 gentle current of carbon dioxide Avas maintained during the heating of 

 the flask, in order to keep air out. The flask was kept immersed in 

 boiling water and before very long a lustrous very thin film of 

 mercury could be seen in the lower part of the condenser-tube, which 

 had become a coating of globules in two hours, when the heating was 

 stopped. The metallic mercury was brushed out of the tube into a 

 capsule and most of the solution in the flask decanted from the 



