242, REporR’T OF THE CHEMIST OF THE 
We desired, however, to carry our work farther, if possible, and 
prove beyond question that there is an actual chemical combina- 
tion between acid and proteid. We desired to prepare the free 
proteid and then treat this with acid and stud'y the results. 
Hammarsten* made a special study of the question of com- 
bination between casein and acids, and concluded that there was 
no ground for believing that a chemical combination takes place 
between casein and the acid used to precipitate it. He based his 
statement on the fact that, by rubbing for several days in a 
mortar with different portions of water a precipitate formed by | 
casein with an acid, he was able to remove the acid so completely 
that the remaining precipitate gave no test for acid. The sub- 
stance which he used in his work was what we have called the 
di-acid. compound of casein. In repeating Hammarsten’s work, 
we are able to remove all traces of acid from the acid-precipi- 
tated casein by the same treatment. We are able by this process 
to prepare a substance free from the acids used as precipitants — 
and also practically free from ash. We found this freshly pre- 
pared substance readily soluble in warm dilute salt solution and 
also in hot dilute alcohol; it also showed the ductile and plastic 
properties given by the substance we called a mono-salt of casein. 
The facts as they presented themselves to us at this point were 
as follows: <A precipitate formed by treating milk-casein with 
an acid, being what we called a di-salt of casein, was by con- 
tinuous washing and trituration converted into a substance free 
from the acid used as precipitant and nearly free from ash; the 
substance precipitated by acid, insoluble in dilute salt-solution 
and in hot 50 per ct. alcohol, was by continuous washing and 
rubbing converted into a substance easily soluble in warm dilute 
salt solution and in hot 50 per ct. alcohol. There was also a 
simultaneous change in other properties. In other words, what 
we called a di-salt of casein was changed into what we had ealled 
mono-salt of casein, but the latter instead of being combined with 
an acid was free from acid and ash. 
In our early study we recognized in a limited way the relation 
of inorganic salts in milk to the neutralization of acid. In Bulle- 
tin No. 214, p. 428, we noticed that when milk, coagulated by 
*Maly Jahresber. d. Thierchem., 7:160 (1877). 
