Milk, as every bacteriologist knows, is not only a universal and excellent food-stuff for human beings, but a medium admirably adapted for the growth and multiplication of microbes. For the latter reason milk deserves every attention at the hands of the hygienist, it being incontestably established that it may serve as a vehicle of disease agents. How well natural milk is adapted to this purpose, viz. to serve as nutritive medium for bacteria, is clear from its alkaline condition, and from its containing all ingredients required for the growth and multiplication of bacteria: proteid, fat, carbohydrate, and a large percentage of the essential mineral matters. It is a matter of common experience that all milk, however carefully it may be collected, however clean and aseptic may be the vessels into which it is received, will on standing, or after being handled in the way usual between collection and distribution, be found to teem with various kinds of bacteria. This fact is confirmed by the bacteriological examination of the milk sold in London shops, milk which, normal though it may be in appearance, chemical analysis, and taste, is usually found to contain hundreds of thousands of bacteria per cubic centimeter; bacteria which belong to various species and some of which when grown separately in sterile milk cause rapid changes and alter profoundly the character of the milk, e. g. Bacillus lactis, Proteus vulgaris, Bacillus coli, Bacillus mesentericus, spores of Bacillus enteritidis, etc. If allowed to stand, the milk containing the above mixture of bacteria exhibits even at ordinary temperatures, but in a more marked degree at temperatures of 70° F. and above, those profound changes which are popularly expressed as ‘going bad,’ changes caused by the rapid multiplication of one or other of the above microbes.