40 METHODS OF QUANTITATIVE ANALYSIS. 



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form. The cream layer is then measured on the scale. 



For whole milk the average is lo to 15 degrees and for 

 half-skimmed milk, 5 to 8 degrees. In 

 order to decide from the reading whether 

 the milk has been skimmed or watered, 

 one must determine the density of the 

 sample thus freed from cream by the 

 method given in the chapter on specific 

 gravity. (See page 22.) 



Of the many forms of apparatus 

 adapted to the optical methods, one of 

 the best is the lactoscope devised by 

 Feser (Fig. 5). A wide glass tube con- 

 tains in the constricted lower en'd a milk- 

 glass cylinder, the wall of which is 4.75 

 mm. from the surrounding tube. At 

 regular intervals on this cylinder, sev- 

 eral equally heavy black bands are 

 glazed. The surrounding tube is pro- 



m li^ vided with a double scale ; the left grad- 



■ iw ~ jIH uated from o to 200 and the right from o 

 to 10. For the determination the tube 

 is filled with the milk sample to the zero 

 mark. Water is then added until after 

 shaking the black lines of the milk glass 

 cylinder just become visible. The per- 

 centage of fats is indicated directly by 

 the right hand scale at the surface of the 

 liquid. 



By the Marchand-Tollens' Lacto- 

 butyrometer (Fig. 6) results are ob- 

 tained more easily but are not so ac- 

 curate. 



Ten c. c. of milk are placed 

 in the narrow glass tube, 



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