PHYSICAL AND CHEMICAL EXAMINATION OF MILK 141 



by addition of mercuric chlorid or potassium bicliromate; both of 

 these preservatives can be purchased in tablet form. Some pre- 

 fer the use of formalin. Mercuric chlorid is the most popular 

 chemical used for preserving composite samples, but it is poison- 

 ous, and therefore some coloring-matter should be added so that 

 the milk has a strikingly different appearance from normal milk. 

 Commercial tablets of bichlorid of mercury have a red coloring- 

 matter incorporated. 



Preservatives should be used in moderate quantity, since 

 large amounts so alter the casein as to render it less soluble in 

 sulphuric acid than normal casein. This is of importance in mak- 

 ing the fat test by the Babcock method. 



In order to obtain a fair determination from composite samples 

 it is important to mix the contents of the bottle with the new 

 sample which is added from day to day. By doing this the 

 cream is prevented from forming a solid surface layer which 

 always militates against accurate work. Furthermore, daily mix- 

 ing is necessary to distribute the preservative thoroughly through 

 the fresh sample. Mixing should be accomplished by a rotary 

 motion, not by violent shaking. 



Before taking the sample for the final test, which should be 

 made at intervals of not more than two weeks, the contents of 

 the bottle should be mixed by a rotary motion, and if the cream 

 adheres to the walls of the vessel or appears to mix with difficulty 

 the milk should be warmed to 40° to 45° C. 



The Specific Gravity 



The specific gravity was formerly thought to be clearly indica- 

 tive of the quality of milk. At present it is considered of httle im- 

 portance in itself, but in connection with other tests it is of great 

 aid in determining the composition of milk. The specific gravity of 

 milk, as has been explained before, is gr.eater than that of water, 

 and the fat counteracts in a measure the effect of dissolved sub- 

 stances. Milk rich in fat has, therefore, a lower specific gravity 

 than poor milk. The specific gravity of cream is below, and that 

 of skimmed milk above, that of whole milk. It is obvious that 

 the removal of fat causes the specific gravity of milk to rise, and 

 that by the addition of water skimmed milk can again be brought 

 to the normal specific gra\aty of whole milk. 



It is clear that the specific gravity is not an accurate index of 

 the quality of milk. It has, however, a definite relation to the 

 percentage of fat and plasma solids. When two of these factors 

 are known, the third one can be calculated. Therefore, if the 

 specific gravity is taken and the fat content determined, the total 



