Page:Pure milk - a lecture delivered in the lecture room of the exhibition, July 30th, 1884 (IA b28525140).pdf/20

 viz.: watering, which will reduce the weight of a gallon; skimming, which will increase the weight of a gallon. Both of these processes are profitable to the milk seller, and a moment's consideration will show that if the milk is carefully skimmed and afterwards watered, the original gravity of the milk will be reproduced so exactly as to deceive any person using the lactometer as a test. It is in this way that a good deal of the watered and skimmed milk is passed off by country dealers to the London vendors and by London vendors to their customers. In one case, however, the lactometer is serviceable, and that is to show that the milk coming from any farm keeps day by day a fairly uniform regularity in quality, and that the milk returned by the milk carriers at the conclusion of their rounds comes back in the same condition as when it went out.

The only reliable and trustworthy method of ascertaining the quality of milk, is by means of a full chemical analysis. To carry this out the water contained in the milk is evaporated; the whole of the solid matters of which I have shown you specimens are left behind in a state in which they can be weighed; the fat contained in these solid matters is then extracted by means of either petroleum or some other suitable liquid, and the solids not fat which are left behind are dried again and weighed. These solids not fat form the real standard by which the question of watering is determined; while the fat which has been extracted when weighed forms the real guide as to whether the milk has been skimmed. If either of these two figures were perfectly constant, one problem of milk analysis would be solved, but unfortunately there is a considerable variation in different samples of milk.

To get over this difficulty a low or minimum figure has been adopted as the standard, so as to allow an ample margin for the natural irregularity of composition. Milk dealers are aware of this difficulty in fixing a standard, and are constantly endeavouring to prove that adulterated milk is really pure milk. 1 here is practically no