( 28G ) 
XIX. — On Milk. By Dr. Augustus Voelcker. 
Milk is a secretion produced from the elements of blood and 
cbjle by the mammary gland of the female animal of the order 
jNIammalia, after giving birth to young. It is a whitish opaque 
liquid, of an agreeable sweetish taste, and faint but peculiar 
odour. It is slightly denser than water. Cow's milk of good 
quality has a specific gravity of about 1030 ; woman's milk, 
1020; goat's and ewe's milk, 1035 to 1042; ass's milk, ]019; 
that of water being 1000. 
Cow's milk and the milk of other herbivorous animals is cither 
neutral, or more generally, when quite fresh, slightly alkaline ; 
the milk of carnivorous animals has always an acid reaction, 
when tested with blue litmus paper. 
Viewed under the microscope, milk appears as a transparent 
fluid, in which float innumerable small round or egg-shaped 
globules, the so-called milk-globules. The fluid constitutes the 
bulk, and the milk-globules but a small fraction, of the milk. 
Completely separated from the milk-globules, the fluid is a 
perfect solution of the following substances : — 
1. Curd, or casein. 
2. Albumen. 
3. Milk sugar. 
4. Mineral matters. 
The milk-globules consist of: — 
5. Thin shells of curd, or casein, enveloping 
6. Fatty matters (the fats of butter). 
Composition and Properties of Curd, or Casein. — When milk is 
allowed to turn acid by keeping for some days, or when any acid 
or rennet is added to new milk, the curd of milk, contaminated 
with more or less butter, separates in the form of a white, flaky 
voluminous substance, having a slightly acid reaction. 
Dried on a water-table, it shrinks greatly in bulk, and becomes 
semi-transparent and horn-like. In this condition it is scarcely 
soluble in water, but dissolves with readiness in a weak solution 
of caustic potash and soda, and is again re-precipitated from its 
alkaline solution by acetic or mineral acids, and restored to its 
former gelatinous condition. 
Casein exists in milk in a state of solution, and is distinguished 
from albumen, which it resembles closely in composition and 
general physical properties, by not coagulating on boiling, and 
by being precipitated by rennet. 
On boiling a solution of casein, it absorbs oxygen, and in con- 
sequence a pellicle, which is insoluble in water, is gradually 
formed upon the surface. A similar pellicle is formed when 
