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THE POPULAR SCIEXCE MONTHLY 



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pump has been called into requisition, wliile a Ligli specific gravity in- 

 dicates a deficiency in cream. A popular form of lactometer, showing 

 directly the proportion of water added to the sample, may be purchased 

 of dealers in chemical apparatus for seventy-five cents, but some care 

 must be exercised in its selection, since one instrument now 

 offered the public has the point marked " skim milk " above 

 that marked "pure milk." Evidently it should be below, 

 as skimmed milk has a greater specific gravity than whole 

 milk. 



In some States the test of specific gravity by the lac- 

 tometer is the only determination made. In spite of the 

 importance of the test, however, it is not always conclusive. 

 A very rich milk will show a suspiciously low specific grav- 

 ity, while a skimmed milk, diluted with water to the proper 

 density and colored to an agreeable cream-tint by "Rich- 

 ardson's Perfected Butter Color " or other dye, will escape 

 so much as the breath of scandal. 



The amount of cream is generally determined by per- 

 mitting the samjile to stand in a graduated jar until the 

 cream separates, and then reading off the volume percent- 

 age directly. Centrifugal machines are also used, but a 

 simpler test than either of these is usually sufficient. If a 

 closed tube, blackened on the inside, is dipped into the 

 sample, and slowly withdrawn, one can judge of the rich- 

 ness of the milk by the opacity of the film remaining on 

 the tube. 



To determine the total solids, five cubic centimetres of 

 the sample of milk are placed in a small platinum dish 

 of known weight, and the joint weight of milk and dish 

 then obtained. The difference between the two will be 

 the amount of milk taken for analysis. It is then evap- 

 orated to dryness on a water-bath without stirring. This 

 will take about an hour, at the end of Mhich time the dish will be 

 found to contain a yellowish-white mass which shows a thin, transpar- 

 ent film on top and a honeycombed structure beneath. A number of 

 cracks will extend throughout the mass, on account of the contraction 

 on drying, but do not indicate that anything is amiss. The dish and 

 contents are then put into an oven at 212° F., and at the end of half 

 an hour are taken out and weighed. The known weight of the dish 

 subtracted from the weight thus obtained gives the weight of the total 

 solids. The ratio of this weight to the weight of milk taken for analysis 

 will give the percentage value. 



If it be desired to find the amount of fat, petroleum-benzine is 

 poured over the residue and is renewed at the end of an hour. After 

 standing half an hour longer, the solvent is again decanted. The resi- 

 due is thoroughly washed with benzine, and is dried in an oven at 



Fig. 2 — 

 Lactometer 

 WITH Direct 

 Graduation. 



