14 



BULLETIN 98, U. S. DEPARTMENT OF AGEICULTUEE. 



hundred and fifty fold. The figure shows graphically what a decided 

 influence the dairyman can exert upon the keeping of his milk by a 

 lowering of temperature only 20°. 



From the foregoing it is obvious that proper refrigeration is of the 



utmost importance in the preservation 

 )f milk. In fact, without thorough cool- 

 ing it is impracticable to keep milk for 

 any considerable length of time in a con- 

 dition that would justify its use for house- 

 W±W$$W<§'$ k°^ purposes. It should be cooled to 

 ■^^^^P* 1 50° F. or below as quickly as possible 



after it is drawn from the cow, as such 



Fig. 2.— Graphical representation of 



the relation of temperature to bac- coo ling will at once check the increase in 



terial growth in milk. bacteria. 



COOLING BY MEANS OF SALT AND ICE MIXTURES. 



Where a small amount of refrigeration is required it is often produced 

 by a mixture of common salt and ice. The action of the mixture in 

 lowering the temperature below 32° F. is as follows : When two solid 

 bodies, as salt and ice, 

 mix to form a liquid a cer- 

 tain amount of heat be- 

 comes latent, called the 

 latent heat of solution. 

 Since this latent heat is 

 taken from the mixture 

 itself the temperature falls 

 correspondingly. The tem- 

 perature obtained by a salt 

 and ice mixture depends 

 principally on the relative 

 proportions of the mixture, 

 and to a less extent on the 

 rate at which the heat is 

 supplied from the outside, 

 the size of the ice lumps 

 and salt particles, and the 

 amount and density of the 

 resulting brine. Hence it 

 is impracticable to give 

 other than approximate temperatures with fixed ratios of salt and 

 ice. From the curve in figure 3 the approximate temperature 

 resulting from different proportions, by weight, of salt and ice may 

 be obtained. If, for example, we desire to know what tempera- 

 ture may be obtained by using a mixture of salt and ice containing 

 15 per cent of salt, follow the vertical line marked "15 per cent of 

































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Ten 



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Fig. 3. — Approximate temperatures obtained with different 

 proportions of salt. 



