79 
attend its more extensive use and abundant production, I 
will first describe its general chemical and physical characters. 
On examining a portion of milk with a powerful micro- 
scope, we perceive that it consists of a transparent fluid, in 
which numbers of minute globules, of about the jo.soo P ar ^ 
of an inch in diameter, are seen to float. It is principally to 
these globules that the whiteness and opacity of the milk is 
owing. On leaving the fluid at rest for a length of time, the 
globules separate in a great measure from the fluid, and rise 
to the surface in the form of cream. This cream, however, 
does not consist entirely of these globules, but always con- 
tains a considerable quantity of the serous portion intermixed. 
It has been found on analysis to contain on an average in 
100 parts 
Butter 4.5 
Curd 3.5 
Whey 92.0 
100.0 
It would appear, therefore, that milk might be divided 
into three distinct substances, namely, the butyraceous, the 
caseous, and the serous ; — the butyraceous being the portion 
contained in the small globules, the principal part of which 
separates on repose ; the caseous being the portion which 
coagulates in the formation of curds ; and the serous being 
the portion remaining in the form of whey and butter-milk. 
In the process of churning the thin film which coats each 
butter globule is oxidised or broken, and the real fatty par- 
ticles then liberated produce an uniform mass. This butter, 
however, is not perfectly free from caseous matter, nor can 
it be rendered so by the process of washing or melting. It is 
the presence of this small amount of caseine, — about sixteen 
per cent., — that renders fresh butter so liable to undergo 
decomposition, — a change which can only be arrested by the 
addition of a large quantity of salt. Butter, when perfectly 
pure, possesses all the characters of ordinary fat, consisting 
