66 BULLETIN" 98, TJ. S. DEPARTMENT OF AGRICULTURE. 
able hygienic conditions, and not to pathogenic (that is, disease- 
producing) bacteria. It is impracticable to reduce the temperature 
of milk much below 50° F. in summer without employing a refrig- 
erating machine or ice ; and as the former is too expensive for the 
ordinary farmer, we are limited to the use of ice or well water. 
Where ice is plentiful and may be had at a nominal cost it is an easy 
matter to reduce the temperature to, say, 40° F., and by referring 
to Tables IV, V, and VI under " Influence of temperature and time 
on the development of bacteria in milk" it will be noted that the 
multiplication of bacteria at this temperature is very small. 
In those locations where natural ice is available it is compara- 
tively an easy matter to cool milk or cream on the farm before carry- 
ing it to the receiving station or creamery. This may be done by 
running the milk or cream over some form of cooler in which cracked 
ice or a mixture of ice and salt is placed, or through which cold 
water is circulated. 
Where the milk or cream is placed in cans and set in cool water, 
or even in a tank filled with ice and water, the cooling goes on very 
slowly, especially if the cans are large. The outside portion, how- 
ever, may be cooled in a comparatively short time, but unless it is 
stirred repeatedly it will take considerable time before the interior 
is cooled down to a point where the development of bacteria is re- 
tarded to such an extent that the milk or cream may be safely car- 
ried to the receiving station or creamery, as the case may be. It is 
often the case that a can of milk is set into a cooling vat in which the 
cooling medium is lower in level than the milk in the can, in which 
case the milk in the lower part of the can may be cooled down to 
approximately the temperature of the cooling medium, while that 
above the level will remain at the higher temperature of the atmos- 
phere; consequently, when the milk is stirred the whole will turn 
sour and spoil. The cold milk, being heavier than the warm, will 
naturally remain at the bottom of the can, while the warmer and 
therefore lighter portion will remain at the top, and practically no 
circulation will take place and the transfer of heat by conduction in 
this case is very slow. 
If proper care is exercised, however, milk and cream may be cooled 
down to a temperature sufficiently low to get to the receiving sta- 
tion or central creamery in good condition by running spring or well 
water through the cooler. In the winter months the lower atmos- 
pheric temperatures assist in the cooling, but in the hot summer 
months the higher temperatures of the atmosphere retard the cool- 
ing; consequently, during the hot weather the milk or cream should 
be run over the cooler very slowly, and if its temperature is not 
sufficiently lowered it should be run over the second time. In this 
