M. C. Lea — Silver Hemisulphate. 327 



As; 



Calculated. 

 78-79 



Found (Mean). 

 70-44 



SO 



1567 



16-33 



PO 



1-98 



2-19 







HO 



3-SO 



1-76 



3-29 



1-78 









100-00 100-03 



This large molecule results from the relatively small propor- 

 tion of P 2 6 , and although the figures obtained for phosphoric 

 anhydride are very concordant, it perhaps is better to consider 

 the substance as a double sulphate. in which part of the sulphuric 

 acid is liable to be substituted by phosphoric. If the silver 

 phosphate is taken as adventitious, the formula becomes sim- 

 ply Ag 4 S0 4 , Ag 2 S0 4 , H 2 0. 



Decompositions. — The action of alkaline hydroxides is con- 

 firmatory of the above conclusions, and also offers further 

 proof of the great stability of the substance. 



When the double salt is placed in contact with excess of 

 dilute sodium hydroxide it blackens, being converted into a 

 mixture of the hemioxide which is intensely black, and the 

 normal oxide. This decomposition, however, takes place 

 much more slowly than with the salts of the protoxide, so that 

 (unless heat has been applied), if after ten or fifteen minutes 

 the alkali is poured off and the oxides are dissolved with 

 dilute nitric or sulphuric acid a considerable residue is found 

 of the red-brown double salt which has escaped decomposition. 



With continued treatment with sodium hydroxide (that 

 obtained from metallic sodium was used as being absolutely 

 free from chlorine, the decomposition is complete. 



The oxide thus precipitated was thoroughly dried at 160°— 

 170° C.j weighed and ignited. Five determinations of oxygen 

 from various specimens gave 4/73 ; 4 -63, and again 4/24. ; 4/19 ; 

 447 per cent of oxygen respectively. A salt with the consti- 

 tution already described should yield one molecule each of 

 hemioxide and of normal oxide, and this mixed oxide should 

 contain 4 - 68 per cent of oxygen. We have then : 



Mean of 5 

 determinations. Calculated. 



Oxygen per cent 4-39 4'68 



The double salt is more readily decomposed by hydro- 

 chloric and hydrobromic acid or even by alkaline chlorides or 

 bromides. Under their action it instantly blackens. That 

 part of the silver that exists in the form of hemisulphate is 

 converted into black hemichloride or hemibromide. The 

 extreme instability of these hemihaloids causes them sponta- 



Am. Jour. Sci.— Third Series, Vol. XL1V, No. 262.- October, 1892. 

 22 



