272 THE CHEMICAL CONTROL OF THE DAIRY. 



seconds that the cream in the pipette takes to flow from the 

 mark above the bulb to that below, which can be determined 

 with sufficient accuracy by any watch with a seconds hand, 

 though it is preferable to use a stop watch. Care must be taken 

 that the cream is free from lumps, or solid particles, and it may 

 advantageously be filtered through muslin. The pipette should 

 be clean and dry, and the cream should be allowed to remain in 

 the pipette a few minutes before making the test, in order to 

 ensure that its temperature is that of the jacket. 



It will be found in practice that it is better to use water of 

 the mean daily temperature in the jacket than water at any 

 constant temperature, because a purchaser is not in the habit of 

 reducing cream to a temperature of (say) 60° F. before passing 

 judgment on its thickness. 



At the same time that the viscosity is estimated a determina- 

 tion of the fat should be made by one of the methods recom- 

 mended (pp. 138, 227, and 237). 



The relation between viscosity and fat can be calculated by the 

 formula given above, which may be expressed, for practical use, as 



log (log V) = 3 log ( o.uF?9^.5 ) + log ^. 

 Or, log (log V) = 2-7 log F -|- log x,. 



A standard viscosity must be fixed, which evidently must be 

 determined by each operator to suit his apparatus, and the con- 

 ditions under which it is necessary to work ; the value of log x 

 or log X, having been found from the determination of viscosity 

 and fat, the percentage of fat which will correspond to the stan- 

 dard viscosity can readily be calculated. 



If a be the percentage of fat found in the cream, and b the 

 percentage of fat which will be contained in the cream of standard 

 viscosity, the cream may be reduced to standard viscosity by 



adding to each 100 parts 100 ( — 5^ — ) parts of separated milk, 



/a — b\ \ o y 



or 100 ( r 7 j parts of milk containing / per cent, of fat. The 



figure 3 '5 may generally be used for / without appreciable error. 

 As an example the following result may be taken : — 



V = 170 

 log V = 2-23045 

 log (log v) = log 2-2304.5 = 0-34839 

 P = 61-4 

 107-521F_ _ 

 0-11 F + 96-5 " ^"^ ^"^ 

 log 63-95 = 1-80584, and 3 log 63-95 = 5-41752 

 then 5-41751 = 0-34839 -|- 5-06913. 



