10 REPORTS ON THE STATE OF SCIENCE.—1918. 
In normal solutions the iodides, nitrates and sulphates are said 
to precipitate at the same concentrations as the corresponding 
chlorides and bromides. The effect of concentration is a little 
obscure in other respects since 5 or 6 drops of normal KCl precipi- 
tate 2 cc. 0°06 per cent. Ag, whereas it takes only 30 drops N/10 
KCl to produce precipitation. The essential thing, from my 
point of view, is that the different univalent cations have different 
liminal values; the difference between hydrogen and lithium is 
greater than that between hydrogen and aluminum. 
From experiments on mastic’? we get the data given in 
Table IT. 
TaBLeE IT. 
Coagulation of Mastic. 
Salt Liminal value, gram Atomic precipitating 
: atoms. power of cation. 
NaCl 1:0 1 
AgNO; 0°125 8 
HgNO; 0-00125 800 
HCl 0:010 100 
CaCl, 0°025 40 
BaCl, 0:°025 40 
ZnSO, 0°050 20 
Al,(SO3)>s 0-0004 2,500 
Al,(NO3)2 0:0004 2.500 
FeCl; 0-0003 3,300 
If we consider the mercury in mercurous nitrate as a univalent 
ion, it is very much out of place, precipitating at much lower 
concentrations than the barium, calcium, and zinc salts. Of 
course, the fotmula should be written Hg,(NO,), with Hg, as a 
bivalent ion.®* In this case the precipitating power becomes 
1,600 instead of 800, which puts it up much nearer the trivalent 
cations than the bivalent ones. The order of cations is: Fe, 
Al>Hg,.>H>Ba, Ca>Zn>Ag Na. Only three anions are 
given in the table, so it is impossible to tell what effect the anions 
have. A good many experiments have been made on mastic with 
different acids, but the degree of electrolytic dissociation varies 
so as to make these results inconclusive. With Prussian blue 
Pappada® found the order of the cations to be: Fe, Al, Cr > Ba, 
Cd > Sr,Ca > H>Cs>Rb>K>Na>Li. Sulphates, nitrates, 
chlorides, bromides, and iodides all behaved alike. Practically 
the same order of the cations was obtained for copper ferro- 
cyanide.’”° In the cases studied by Pappada the specific adsorp- 
tion appears to play a very small part. The data for arsenic 
66 Pappada. Gazz. chim. ital. 42, I, 263 (1912). 
87 Freundlich, Kapillarchemie, 367 (1909). 
68 Oge, Zeit. phys. Chem. 2'7, 285 (1898). 
69 Zeit. Kolloidchemie, §, 83 (1911). 
79 Pappada, Zeit. Kolloidchemie, 9, 136 (1911) 
