METHODS OF CREAM-RAISING. 117 



By a unit of heat is meant the amount of heat which is necessary to 

 raise 1 Ib. of water one degree from the melting point of ice, that is, from 

 to 1 C. The quantity of heat which will raise 1 Ib. of water at any 

 temperature one degree, or, vic& versd, the quantity which must be removed 

 from 1 kilogram of water in order to reduce its temperature one degree, is so 

 similar in amount to that amount of heat which we have just described as 

 constituting a unit of heat, that it may be regarded as the same. According 

 to De la Provostaye and Desains, and Regnault and Petit, the latent heat 

 of water may be taken at 79*25, or, roughly speaking, 79 units of heat on 

 the Centigrade thermometer. In order, therefore, to convert 1 Ib. of ice 

 at C. into water at C., as much heat is required as will convert 1 Ib. 

 of water at C. to 79 C., or to raise 79 Ibs. of water at any temperature 

 1 C. 1 Ib. of water at 79 C. will be reduced to C. by 1 Ib. of ice 

 after the ice has been melted, or will cool by one degree 79 Ibs. of water 

 of any temperature. Vice versd, I Ib. of ice at C. in melting cools down 

 1 Ib. of water at 79 C. to C., or will reduce 79 Ibs. of water at any 

 temperature by one degree. In these statements no account is taken of 

 the loss or gain of heat due to surroundings. 



The specific heat of milk of average chemical composition water being 

 taken as 1 is, as was stated in 4, about '85. In order to cool milk, 

 therefore, there is required only 85 per cent of the quantity of ice that 

 would be required to cool an equal quantity of water. 



The question whether it is economical and desirable to use ice-manu- 

 facturing machines in dairies has not been properly investigated. According 

 to M. Schrodt's experiments, it would seem profitable to use such machines 

 in very large dairies in towns where ice is unusually expensive to procure, 

 but certainly not in small dairies, or in dairies which can obtain their ice 

 cheaply. 



53. Methods of Cream-raising. Before the days of milk-centri- 

 fugal machines, and while the old methods of cream -raising were being 

 perfected, the merits of different methods were often attempted to 

 be tried by comparative tests. In Denmark this was attempted to 

 be done by working on milk of the same origin, churning the cream 

 separated, determining the yield of butter, and regarding as most 

 suitable the method which yielded the largest quantity of butter. 

 This method, although somewhat cumbersome and involving many 

 inaccuracies, had the advantage of not requiring chemical investiga- 

 tion. It is not suited, however, for reliable comparison. The 

 author preferred for this reason, in his comparative experiments, 

 which were likewise carried out on milk of similar quality, to 

 determine the percentage of fat in the milk and the skim -milk, as 



