New York .\GRICULTURAL EXPERIMENT STATION. 203 
with more delicate means of measuring the amount of acid that 
might combine, it was found that some portions of the former 
experimental work were too imperfect to afford basis for definite, 
quantitative conclusions. As a result of our latest more complete 
and accurate work, it has been established that casein, while capable 
of removing acid from a surrounding solution, does not form any 
simple, definite combination with acids. The amount of acid taken 
up by casein is not fixed and definite but varies (1) with the con- 
centration of the acid, (2) with the duration of contact until 
equilibrium is reached, (3) with the degree of agitation, (4) with 
the temperature, and (5) with the kind of acid. The simplest and 
most satisfactory explanation of the facts is that when acids act 
upon casein, no definite chemical combination occurs, but the acid 
is simply adsorbed by the casein; adsorption meaning the process 
whereby a solid substance A (like casein) in contact with a solution 
of a dissolved substance B (dilute acid) concentrates B upon its 
surface, withdrawing a portion of B from solution without form- 
ing with it a definite chemical compound. This conclusion may pos- 
sibly be modified by more exhaustive work, but the data upon which 
it is based are far more complete and satisfactory than any others 
that have been contributed so far to the study of this question. 
(2) Soluble products formed by action of acids on casein and 
paracasein.— Casein and paracasein are very easily soluble in acids, 
even in dilute acids. The ease with which dilute acids dissolve 
casein was not appreciated until our recent work was published 
(Technical Bul. No. 3). Acids of = concentration dissolve con- 
siderable amounts of casein at ordinary temperatures and much more 
at 45° C. Acids as dilute as {2; and 5, dissolve appreciable 
amounts of casein at 45°C. It is probable that the dissolved product 
is the result of definite chemical combination between casein and 
acid. 
(3) Preparation of calcium-casein and paracasein compounds.— 
Base-free casein can be prepared by treating diluted skim-milk with 
acids, removing the acid and inorganic matter by repeated washing 
and filtration. This uncombined protein, casein, when triturated 
with calcium carbonate suspended in water, or when dissolved in 
lime water and then treated with an acid until the solution is neutral 
to phenoephthalein, combines with about 2.40 per ct. of calcium 
oxide, forming a compound known as basic calcium casein. By 
treating base-free casein dissolved in lime water with acid until the 
reaction is almost neutral to litmus, there is found a compound of 
