274 REPORT OF THE CHEMIST OF THE 
INTRODUCTION. 
In connection with our study of the action of acids upon 
milk-casein (calcium casein), it occurred to us that the results 
might have an application in some of the stages of butter- 
making. We have clearly established the fact that two distinct 
substances are found in succession when calcium casein is 
brought into contact with an acid, whether it be the lactic acid 
produced in milk by the fermentation of milk-sugar, or some 
other acid such as acetic, hydrochloric or sulphuric. When a 
small amount of acid is added to milk or to a preparation of — 
calcium casein, a precipitated substance unlike calcium casein is 
formed, this substance being soluble in warm 5 per ct. solution 
of sodium chloride and also in hot 50 per ct. alcohol, and possess- 
ing characteristic properties of plasticity and ductility. This 
substance we were at first led to regard as a compound formed 
by direct combination of casein and acid and we called it a casein 
mono-salt of the acid used as precipitant; but we have recently 
shown (Bulletin No. 261) that the compound. is base-free casein. 
When this body is treated with an additional amount of acid 
a substance is formed which is insoluble in warm 5 per ct. salt 
solution and nearly so in hot 50 per ct. alcohol and which has 
lost the plastic and ductile properties that characterize free 
casein. 
This substance we formerly regarded as a casein di-salt of the 
acid used to form it, in the belief that there were two series of 
precipitated salts formed by casein with an acid. Since our later 
work shows that there is only one series, this substance we now 
regard as simply a casein salt of the acid used as precipitant. 
The compound we formerly designated as casein monolactate 
is free casein or simply casein, and the body we previously called 
casein dilactate is casein lactate. 
Since the presence of one or both of these compounds is neces- 
sarily involved in all operations where milk undergoes the change 
of ordinary souring, it seemed desirable to make a special 
study of the following points: (1) What, if any, relation these 
compounds might have to the ripening of cream, preliminary to 
butter-making; (2) whichs of these compounds is commonly 
present in butter and buttermlik; and (3) whether the occur- 
