METHODS OF EXAMINATION 201 



is made, tlie milk should be well mixed. In milk that has 

 been thoroughly chilled, it is difficult to again mix the 

 cream equally, so one must be very exact in taking the 

 sample. It often happens, in the milk shops, that the 

 milk is not well mixed when sold and an examination 

 of what is left may show so low a percentage of fat that 

 one immediately becomes suspicious that adulteration 

 has taken place, although it may not be so. 



6. Preserving the sample for analysis. If the milk sam- 

 ple is to be kept or shipped, it is necessary to add a pre- 

 servative. For this, potassium dichromate (| grm. to 1 

 liter) is used, which keeps the milk fluid for a long time ; 

 this addition causes an increase in the specific gravity 

 and the result of the chemical examination is also af- 

 fected. This must be taken into consideration at the 

 final test. It is generally better to add 20 drops of forma- 

 lin to every liter, yet in this case it is possible that the 

 specific gravity of the whey may be affected, on account 

 of a partial splitting of the casein. It is still better to boil 

 the filled bottle and then close it with a sterilized cork. 

 In taking the sample, it is also necessary to avoid adding 

 water even in the slightest quantity (rinse the bottles 

 with milk), in order that no nitrate shall be added. 



c. The herd sample. In cases in which adulteration is 

 suspected, comparison may be made of the milk under 

 suspicion with that of the herd milked under super- 

 vision. The importance of the stable, or herd test has 

 been much overestimated. The great variations in the 

 composition of the milk of individual animals, leads one 

 to be careful in putting confidence in a comparison of the 

 fat content of milk on different days ; but comparisons 

 may fairly be made between the quantities of solids not 

 fat and the specific gravity of the whey, which are fairly 

 regular. The stable or herd test is very uncertain, 

 therefore, for individual cows, or small herds, but it may 



