EXPEEIMENT STATIOIST. 81 



Tests of the quality ofmilJc. — Various methods have been pro- 

 posed for testing the quality of milk. Of them all, there are but 

 two which are to be depended upon ; one of these is the specitic 

 gravity (or density) test, the other, chemical analysis. 



The specific gravity test is the one commonly employed in the 

 control of market milk, by Boards of Health and police author- 

 ities. Milk is, bulk for bulk, slightly heavier than Avater. A 

 vessel that will contain 1000 grains of water will hold 1029 to 

 1034 grains of milk. The lactodensimeter, commonly called the 

 lactometer, is a glass spindle with a slender stem marked off into 

 degrees which sinks in milk to diflFerent depths according to the 

 specific gravity of the liquid, which can be read from the scale. 

 This instrument, properly constructed and skillfully used, gives 

 the specific gravity, with great accuracy, by a single observation. 



Milk is made heavier than water by those of its ingredients, 

 which are heavier than water, namely : casein, albumin, sugar and 

 ash. The fat of milk is lighter than water, and tends to reduce 

 its specific gravity, so that the specific gravity of milk is dimin- 

 ished by adding water and increased by removing fat. Unless, 

 therefore, milk is falsified both by watering and skimming, the 

 specific gravity serves to indicate very exactly its genuineness 

 and its richness. 



If milk is watered to any considerable extent, its specific grav- 

 ity is brought below 1.029 and the lactodensimeter promptly 

 detects the addition. If, however, the milk first be skimmed or 

 the top poured away after some hours' standing, the skimmed or 

 bottom milk will show a high specific gravity and a considerable 

 addition of water may be made without reducing the specific 

 gravity below that of pure milk. Such double falsification of 

 milk requires for its detection that the sample shall be let stand 

 for cream or that chemical analysis shall be resorted to. 



The test by chemical analysis is the only absolutely conclusive 

 means of ascertaining the quality of milk, A determination of 

 the solids of milk by evaporating off the water from a known 

 quantity and weighing the residue, will decide whether milk has 

 been watered or the cream removed, and combining both of these 

 frauds only makes the detection of them more certain. If we 

 separate and weigh the fat, the casein and the sugar of a sample 

 of milk, we get an accurate notion of its composition. Ordina- 

 rily the determination of the solids and fat suffice for all practical 

 purposes. 



