Milk. 
297 
well as more rapid, in shallow vessels. There is another reason 
for preferring shallow vessels. Milk as it comes from the cow 
has a temperature of about 90^. If kept in this condition for 
any length of time, air being freely admitted, it rapidly turns 
sour. Hence it is of consequence to reduce it as rapidly as pos- 
sible down to a temperature of at least 60° Fahr. In a shallow 
tinned-iron milkpan placed upon stone this change is soon 
effected ; and then, in a good dairy, the milk may be kept from 
36 to 48 hours, at a season when in deeper vessels it would 
soon turn sour. When once begun, the process of acidification 
cannot be stopped by any available means. Hence it is of great 
importance to cool down the milk as rapidly as possible. As 
metals are good conductors of heat, shallow tinned-iron milk- 
vessels, resting on stone, are better adapted to keep milk sweet 
than glass or earthenware, or slate-pans, placed on a bad con- 
ductor like a wooden bench. 
It must not be imagined, however, that the lower the tempera- 
ture is allowed to sink the more cream will rise ; for we must 
bear in mind that with the reduction of the temperature the 
specific gravity of the liquid is raised, and the rising of the cream 
or milk-globules checked accordingly. 
When shallow metallic milk-vessels are employed in a 
proper dairy, kept at this temperature, all the cream that will 
rise at all will have come to the surface in about 24 hours. 
Under these circumstances it is therefore no use to set milk aside 
for a longer period. Some people let milk get sour before they 
skim it ; but although the layer of cream in that case appears 
more bulky and of greater consistency, it does not produce so 
much nor so good a quality of butter. On this point we possess 
an interesting experiment by Sannert, who put aside two equal 
quantities of milk, of which the first skimmed after 30 hours 
yielded 30 lbs. of butter ; and the second skimmed after a lapse 
of 60 hours, only 27 lbs. of butter. 
In another experiment two equal quantities of milk yielded — • 
the one when skimmed' after 30 hours, 31 lbs. of butter ; and. 
the other after 60 hours, 29 lbs. of butter. In both experiments, 
in which the milk was skimmed after 30 hours' standing, the 
skimmed milk was still sweet, and the cream not so thick and 
less in bulk than that which was thrown up after 60 hours' 
standing. 
Composition of Cream. — As may be expected, the composition 
of oream varies greatly, according to the circumstances under 
which it is produced. Four different samples analysed in my 
laboratory yielded the following results : — 
