26 ATOMIC WEIGHTS OF SODIUM AND CHLORINE. 



that this solubility steadily diminishes, without a very sharp change 

 at any time, as the precipitate remains in contact with the liquid. 



A variable solubility of this nature may be due to one of two 

 causes. Kither the precipitate is capable of assuming two or more allo- 

 tropic forms, of which the least stable appears at first,* or else it 

 separates in a finely divided state, and gradually becomes aggregated 

 into larger masses. The work of Ostwaldf and of Hulett % has shown 

 how greatly a difference in size of the grains may affect the solubility, 

 especially of a salt only slightly soluble. 



The latter explanation seems the more probable for several reasons. 

 In the first place, no sharp change in the solubility has ever been 

 detected, such as would be expected from the appearance of a new, 

 more stable solid phase. Again, the difference between the specific 

 gravity of the flocculent precipitate and fused horn-silver is very 

 slight, and their heats of solution in cyanide are identical, indicating 

 little or no difference of molecular constitution between the two.|| 

 Moreover, as will be shown later, the curdy precipitate may be success- 

 fully washed in a measure unattainable with crystalline powders, show- 

 ing that the innermost recesses of the solid are more than usually 

 accessible, and therefore that the structure is i very finely divided and 

 loosely knit. The more compact, less soluble precipitate, obtained 

 either by heating or by long standing, has largely lost this possibility 

 of being thoroughly washed, and has become also much less soluble 

 facts which are quite in line with the above hypothesis. However this 

 may be, the uncertainty of the solubility is a fact of important signifi- 

 cance to the analyst, for it obliges him to determine the amount of 

 solution in every experiment the solubility can never be assumed as 

 known. 



The matter is yet further complicated by the well-known fact 

 that the presence of excess of either ionized silver or ionized chlorine 

 causes a depression of the solubility, in the manner demanded by the 

 law of concentration effect. A large excess of either precipitant 

 reduces the solubility to so small a value as to be practically negligible 



*Ostwald, Z. phys. Ghem., 22, 307, (1897). 



fOstwald, Z. phys. Chem.,34, 495 (1900). 



JHulett, ibid,37, 385 (1901) ; 47,357 (1904). 



|| Laevinsohn determined the specific gravities. His results were verified and 

 the ithermo-chemi'Cal argument added to them by Dr. W. N. Stull and one of us, 

 in some work not yet published. 



JHoitsema, Z. phys. Chem., 20,272 (1896). 



