10 MILK ANALYSIS 



the lactometer, a delicate and accurately graduated hydrometer. 

 The instrument must be immersed carefully so as not to wet 

 the stem above the point at which it will rest. Its accuracy 

 should be verified by immersion in distilled water at 15.5° and 

 milks of known specific gravity. 



More accurate determinations may be made with a balance. 

 A special form, the Westphal balance, is adapted to the deter- 

 mination of specific gravity only, the weights being so arranged 

 that a simple enumeration of them gives the gravity directly. 

 The cheap forms of this instrument are not satisfactory, but 

 some made by German houses are excellent. The ordinary 

 analytic balance may also be used. A plummet consisting 

 of a thick glass rod or short sealed tube, weighted with mer- 

 cury, of a bulk of about 10 c.c. is suspended from the hook of 

 the balance by means of fine platinum wire and the weight 

 ascertained. It is then submerged in distilled water and the 

 weight also noted. The water is contained in a narrow up- 

 right cylinder resting on a bench or support above the scale 

 pan. The loss of weight of the plummet is, of course, the 

 weight of the bulk of water that it displaces. The specific 

 gravity of any sample can be determined by weighing the plum- 

 met immersed in the sample and dividing the loss in weight 

 by the loss in water. The quotient is the specific gravity. 



The specific gra^'ity bottle is not convenient for milk on ac- 

 count of the liability of the upper layer of the liquid to be richer 

 in fat than the lower; the overflow, therefore, does not repre- 

 sent the mixture. 



Total Solids. — This determination may often be made 

 with sufficient accuracy for practical purposes by evaporating 

 a measured volume (e. g., 3 or 5 c.c.) in a shallow nickel dish 

 from 5 to 8 cm. in diameter. Nickel crucible-covers are suitable. 

 The thin glass (Petri) dishes used for microbe culture are con- 

 venient. When greater accuracy is required, and especially 

 when the ash is to be determined, platinum dishes must be 



