252 REPORT OF THE CHEMIST OF THE 
base-free proteid we add a dilute acid, another body appears to 
be formed, which is insoluble in dilute salt solution and in hot 
50 per ct. alcohol and which differs also in a marked loss of the 
plastic properties exhibited by the base-free proteid. This is the 
familiar substance ultimately formed when milk is coagulated by 
ordinary souring or by direct addition of acids in sufficient quan- 
tity. The formation of this substance has usually been explained 
in two different ways: (1st) It has been quite generally held that 
the acid unites with the inorganic portion (calcium) of the milk- 
casein, thus destroying the combination, and that the free pro- 
teid then appears as solid. (2d) It has been held by some 
that the acid actually combines with the proteid, forming a 
casein salt of the acid used. According to the first explanation, 
only one substance is formed when milk-casein is treated 
with an acid, forming a precipitate, and this one substance is 
the base-free casein. According to the second explanation, two 
substances are formed, one after the other, by treating milk- 
casein with an acid, viz.: (1st) the base-free casein and (2d) a 
compound formed by the combination of the base-free casein 
with the acid. 
Now, the existence of one body with two different sets of 
properties, or of two different bodies, differing in their proper- 
ties, must be acknowledged, when we treat milk-casein with an 
acid in proper proportions. When, by treating calcium casein 
with a certain amount of acid, we obtain the base-free proteid 
and then, by treating this with an additional amount of acid, we 
obtain a body differing in properties from the base free proteid, 
the difference must either be due to chemical combination of 
the proteid and acid or else be the result of a purely physical 
change caused by the acid. Contrary to the view generally held, 
we have believed that there is actual combination between the 
proteid and acid, forming a casein salt of the acid, and this we 
formerly called a casein di-salt of the acid used. Since we have 
shown that our supposed casein mono-salts of acids are simply 
the base-free casein, it would appear that there is only one series 
of casein compounds formed by combination with acids, existing 
as a precipitate. 
Hammarsten held that there could be no combination between 
the casein and acid, because by trituration with water in a mor- 
