32 BUTTER-MAKING. 
reliable. The fat-globules themselves are said to be almost 
transparent, yet the color and opacity of milk is largely due 
to their presence. This characteristic may perhaps be explained 
by assuming that the fat-globules in milk deflect the light 
instead of allowing it to pass through them. 
The opacity of milk, after the fat has been removed, is 
due to the presence of nitrogeneous matter. After the fat 
has been removed from the milk, the milk still continues to 
be opaque. When the albuminoid matter has been removed 
and filtered off the filtrate becomes clear and transparent. 
Chemical Reaction of Milk.—Milk when fresh shows an 
amphoteric reaction, which means that it exhibits both an 
alkaline and an acid reaction when tested with litmus paper. 
It turns blue litmus paper red, and red litmus paper blue. This 
peculiar behavior of milk is said to be due to the caseous matter 
in the milk, which itself has an acid reaction, but the remainder 
of the serum has a slight alkaline reaction. By testing the 
reaction of fresh milk with a tenth normal alkali solution, 
and using phenolphthalein as an indicator, it will be found 
to give an acid reaction. After standing, milk soon becomes dis- 
tinctly acid, which is due to a change of the milk-sugar into 
acids, chiefly lactic acid, through the action of micro-organisms. 
Richmond maintains that the amphoteric reaction of milk 
has acquired a false importance, as he believes that the neu- 
trality, as measured by the action of litmus paper, is not chemi- 
cal neutrality. 
Specific Gravity of Milk.—By specific gravity of milk we 
mean the weight of the milk as compared to that of an equal 
volume of water at the same temperature. If a certain volume 
of water weighs 1000 pounds, an equal volume of milk at the 
same temperature and under the same conditions, will weigh 
about 1032 pounds. Reducing the figure to a basis of 1, as is 
always done, the comparison between the two equal volumes 
of water and milk will be 1 and 1.032. This latter figure 
represents the average specific gravity of normal milk. 
It can be readily seen that the correct specific gravity can 
