New York AGRICULTURAL EXPERIMENT STATION. 27 
qt 
Lhe possibility of the phenomena belonging to the intricate border- 
land of adsorption combinations must not be lost sight of.” 
The selective action of casein in taking acids but not neutral 
mineral salts from solution is paralleled by that of charcoal. 
Freundlich (loc. cit.) finds that a carefully purified preparation of 
the latter adsorbs mineral and organic acids, particularly the latter, 
but alters the concentration of mineral salts by less than one per ct. 
In this case the adsorbent is equally inert in a chemical sense to- 
wards either acids or salts. The reaction is purely one of surface 
energy, independent of any possibility of chemical combination be- 
tween adsorbent and dissolved substance, and the same may be the 
case with casein. 
It appears, however, that in all cases adsorptive power and the 
tendency to react chemically are not so sharply separated. van 
Bemmelen! says: ” If P and C (adsorbent and dissolvent substance ) 
are indifferent to each other according to the ordinary chemical con- 
ception, an adsorption may occur, but it is in many cases only a 
weak one.” He found that the hydrogel of SiO, adsorbed strong 
bases much more strongly than strong acids or salts. MnO,, which 
kas acid properties and can combine chemically with strong bases, 
adsorbs alkali better than it does acids or salts. Metastannic acid, 
which can form combinations with acids, adsorbs potassittm chloride 
weakly, hydrochloric acid strongly. From these and other similar 
facts, van Bemmelen concludes that it appears that “in many cases 
adsorption is distinguished as the forerunner of chemical combi- 
nation.” Adsorption is strong in many cases when, under other 
conditions or by gradual inner changes, chemical compounds can be 
formed. For example, van Bemmelen found? that colloidal silica 
- in contact with barium hydroxide adsorbed the latter in accordance 
with the criteria of adsorption, so long as barium hydroxide of less 
than a certain concentration was used. When this concentration 
was exceeded, combination occurred and a compound having the 
composition BaSiO,. OH,O separated in crystalline form. In 
double transposition in solution, as between salt and salt, the new 
compound is often precipitated as a colloid and changes later into a 
. 
*Ztschr. Anorgan. Chem., 23:342. 
*Ztschr, Anorgan. Chem., 36:380. 1903. 
