PHYSIOLOGICAL CHEMISTRY 331 



(2) The Percentage of Protein. Three grs. of milk are diluted with four 

 times its volume of distilled water, a few c.c. of a solution of sodium chloride are 

 added, and then a solution of tannic acid until all the protein has been precipi- 

 tated. The precipitate is filtered off through an ash-free filter paper, and 

 thoroughly washed with distilled water. The filter paper with the precipitate 

 is removed to a Kjeldahl's combustion flask, and the nitrogen estimated as 

 described on p. 410. The result multiplied by 6 '37 gives the total amount of 

 protein contained in the sample of milk. 



(3) The Percentage of Fat. The dietetic value of a milk depends to a 

 large extent on the amount of fat it contains. There are, therefore, numerous 

 methods employed for the quantitative estimation of this, some of which are only 

 approximate. The following method (Adam's) will be found very simple and 

 sufficiently accurate for most purposes : 



Measure 5 c.c. milk and drop it on to a strip of Adam's fat-free porous paper ; l 

 allow this to dry in the air bath at 60 C., then roll it up and place it in the 

 extractor of Soxhlet's apparatus (see p. 313). The weight of the distilling flask is 

 ascertained before beginning the extraction, and then again after the extraction 

 has been allowed to proceed for about one hour and the ether has been distilled 

 off; the increase of weight gives the amount of fat in 5 c.c. of milk. Sufficient 

 ether should be used to fill the Soxhlet one and a half times, and it should be 

 made to siphon over at least twelve times. 



(4) The Percentage Of Sugar. Ten c.c. milk are mixed with twice that 

 amount of alcohol (meth. spt. ) so as to precipitate all the protein, which is then 

 filtered off. The precipitate is thoroughly washed with alcohol, and the washings 

 are then placed on the water-bath till all the alcohol has evaporated. The 

 contents of the evaporating basin are then carefully washed into a 100 c.c. 

 measuring cylinder, and the volume made up to 100 c.c. This is then placed in 

 a burette and titrated with boiling Fehling's solution, as described on p. 45.0 

 Ten c.c. Fehling's solution correspond to 0'0676 g. lactose, therefore the number 

 of c.c.'s of the diluted extract required contains 0'0676 grm. lactose. In order to 

 calculate the percentage it must be remembered that each c.c. of the solution in 

 the burette corresponds to O'l c.c. of the original milk. 



(5) The Percentage Of Ash. A weighed quantity of milk is evaporated to 

 dryness on a water bath in a weighed crucible. The crucible is carefully heated 

 over a free flame until a perfectly dry and black ash has been obtained. The 

 flame is now strengthened and the ash is heated until it becomes white. The 

 crucible is then allowed to cool in a desiccator, after which it is weighed. 



CHAPTER VIII. 

 BLOOD. 



To the unaided eye, ordinary vertebrate blood appears to be a homo- 

 geneous red fluid, but microscopical examination shows that the red 

 colour is really due to certain formed elements, the red corpuscles, 



1 The paper can be obtained from any of the dealers. 



