276 REPORT OF THE CHEMIST OF THE 
‘chemical compound of crystalline form and definite composition. 
It is questionable whether the colloidal precipitate first formed is a 
chemical compound of definite proportions rather than an adsorption 
compound, or a transition between the latter and a definite chemical 
compound. van Bemmelen has determined the transition of pre- 
cipitated colodial adsorption compounds of Al?O,, BaO, and CuO 
into chemical compounds. 
From the standpoint of van Bemmelen discussed in the preceding 
paragraph, viz., that a chemical affinity between two substances, 1n- 
sufficient to cause a chemical reaction under existing conditions, 
may result in the formation of an adsorption compound, which may 
or may not give rise to a definite chemical compound, one may 
regard the action between casein and acids, by which insoluble acid- 
casein with varying content of acid is formed, as the formation of 
an adsorption compound under such influence; and the change to 
soluble acid-casein formed on long standing, on warming or on con- 
tact with acids above a certain degree of concentration, as the change 
of the adsorption compound into a chemical one, such as a proteid- 
acid salt. 
Assumptions aside, however, as to the possible relation between 
the basicity of casein and its adsorptive power, and as to the nature 
of soluble-acid-casein, the action of dilute acids may be briefly 
stated thus: The acids first concentrate upon the casein by ad- 
sorption and then begin to dissolve it more or less slowly according 
to conditions of temperature concentration, etc.: The dissolved sub- 
stance, at least in the case of hydrochloric acid, contains more acid 
than the adsorption compound. 
The fact that acid-casein does not dissolve in warm 5 per ct. salt 
solution, as found by Van Slyke and Hart! is explained by the 
work of Hardy (loc. cit.) who found that the presence of acids de- 
creases or inhibits the solvent power of salt solutions towards pro- 
teids. When the insoluble acid-casein is treated with salt solution, 
the latter would naturally extract part of the adsorbed acid, and 
largely lose its solvent power. 
Although the results obtained during our limited time of investiga- 
tion in this line of work are not complete, the facts that have been 
4 
"N. ¥. Agr. Ex. Sta,, Bul. 261. 1905, 
4 
