Milk. 
299 
quence of which their light fatty contents, liberated from the 
denser casein-shells, rise to the surface with greater facility, and 
there occupy less room than the unbroken milk-globules, w hich, 
on account of their greater specific gravity, are more sluggish in 
rising. 
In my experiments the milk had to be measured out several 
times over, in order to secure accuracy ; and this exposure to 
extra agitation explains the unusually small bulk of the cream : 
since in all instances of careful experiment the milk must be a 
good deal agitated, until it becomes a sort of mixture of ordinary 
v:ream and ready-made butter. The large quantity of butter 
yielded by cream in Mr. Horsfall's and my own experiments is 
explained by this circumstance. We may learn from this that 
<in farms where cream is sold the milk should not be shaken 
more than is absolutely necessary. 
Although no doubt originally all the butter exists in the shape 
of butter-globules, a partial separation, however, between the 
contents and the envelopes of these take place already in the 
udder of the cow ; for if ether, an excellent solvent for unencased 
fatty mattei-, is carefully mixed with the milk as it comes from the 
cow, and the layer of ether which collects after some time on the 
surface of the milk is evaporated to dryness, an appreciable 
quantity of fatty matter is left behind. This is not more than 
might be expected ; for the milk in the udder of the cow to some 
-extent becomes agitated by every violent movement of the ani- 
mal. For this reason, whether bui;ter or cheese be our object, 
we should endeavour to keep milking-cows as quiet as possible. 
Composition of Skim Milk. — As cream-globules are lighter than 
milk their removal must increase the density of the remaining milk. 
Good new milk, on an average, has a specific gravity varving 
from 1'030 to 1032. If it is rather poor in cieam, but not diluted 
with water, its specific gravity rises a little ; and if it is very rich 
in cream, the gravity of the milk will sink to 1-029 or 1-028. 
I have carefully determined the specific gravity of the two 
samples of skim-milk, the analysis of which is stated below. At 
62° Fahr. the first sample had a specific gravity of 1-037 ; and 
the second sample, at the same temperature, a gravity of 
Composition of S/iim-7nil/;. 
No. 1. . No. 2. 
Water 89-65 89-40 
Butter (pure fat) -79 -76 
Casein 3-01 2-94 
iMilk-.sngar 5-72 6-05 
Mineral matters (ash) -83 -85 
100-00 100-00 
'Containing nitrogen 'iS '47 
