6 



BULLETIN 420, U. S. DEPAETMENT OP AGEICULTUEE. 



The bottles placed in still air were cooled by natTiral circulation 

 brought about by the difference in temperature between the compara- 

 tively warm air in contact with the outer surface of the bottles and 

 that at a distance. As the air in contact with a hot surface, such as 

 that of the bottles containing milk, becomes heated, it rises and its 

 place is taken by cold air; hence, the rate of circulation of the air 

 depends upon the difference in weight of the respective columns of 

 warm and cold air. As this difference in weight is very shght, the 

 circulation is naturally slow, and hence a slow rate of cooling follows. 



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Fig. 2. — Cooling of milk and water by forced air.— Difference in the rate of eooluig of nulk and water when 

 exposed to an air blast. Quart bottles. 



In the case of forced circulation of air, however, the fihn or sleeve 

 of hot air surrounding the vessel is broken up, and a large volume of 

 cold air is brought into direct contact with the outer surface of the 

 container; the cooUng therefore goes on very much faster. The 

 forced circulation of the air in these experiments was made by a 16- 

 inch electric fan, which produced over the bottles an air velocity of 

 approximately 1,250 feet a minute, measured by an anemometer 

 which had just been calibrated by the Bureau of Standards. 



It will be noted by reference to figure 3 that it required 4^ hours 

 to reduce the temperature of the milk in the pint bottles when placed 

 in still an from 142° to 50° F. (61.1° to 10° C.) with a practically con- 

 stant temperature of 40° F. (4.4° C.) inside the room. When the air 



