Theory of Deep Setting System 169 
ture of about 40° F., and held at that temperature 
for twelve to twenty-four hours, not only could 
the depth be increased from 4 to 20 inches, but the 
separation was much more complete in a shorter 
time. In order to bring about quick cooling, ves- 
sels not more than a foot in diameter were used, and 
water, either from cold springs or containing ice, 
was used as the refrigerant. Two essentials, then, 
for complete creaming by this system are the rapid 
and immediate cooling of the milk to 40° F., and 
a sufficient supply of ice to maintain this tempera- 
ture for twenty-four hours. Under these condi- 
tions the fat may be so completely separated that 
not more than .2 of 1 per cent of fat is left in the 
skimmed milk. Various devices have been used, 
the best known of which is the Cooley system, in 
which the cans are not only surrounded by ice- 
cold water, but completely submerged in it,. the 
chief effect of the submerging being to guard the 
milk against contamination through the atmosphere. 
The same conditions as to the character of the milk 
affect completeness of separation in both. the shallow 
pan and deep setting systems, but no completely 
satisfactory explanation has ever been assigned as a 
reason why the fat globules should rise more 
rapidly and more completely through 20 inches of 
milk at a temperature of 40° F. in the deep setting 
system, than through 4 inches at a temperature of 
60°-70° F. in the shallow pan system. : 
It has been asserted that because water is a 
better conductor of heat than fat, it will cool more 
rapidly than the fat, and will increase in density 
