_Kinds of Bacteria in Milk._ It is clear from the foregoing that the only valuable estimation of bacteria in milk is a qualitative one. The kinds commonly found may be classified thus: 1. Non-pathogenic; fermenting and various unclassified micro-organisms. 2. Pathogenic; tuberculosis, typhoid, cholera, scarlet fever, diphtheria, and suppurative diseases have all been spread by the agency of milk. 1. _The Fermentation Bacteria_ At the most we can make a merely provisional classification of these processes. Many of them are intimately related. Of others, again, our knowledge is at present very limited. It may be advisable, before proceeding, to consider shortly what are the constituents of milk upon which living ferments of various kinds exert their action. A tabulation of the chief constituents would be as follows: { (1) Water 87.5 per cent. Ordinary { (2) Milk-sugar 4.9 " " fresh milk = { (3) Fat 3.6 " " 100 per cent. { (4) Proteids (casein, etc.) 3.3 " " { (5) Mineral matter 0.7 " " ----- 100.0 Another mode of expressing average milk constitution would be thus: Fat 4.1 per cent. Solids not fat 8.8 " " ---- 12.9 " " It is probably too obvious to need remark that milks vary in standard, but the above figures may be taken as authentic averages. _Milk-sugar, or Lactose_ (C_{12}H_{24}O_{12}). This is an important and constant constituent of milk. It forms the chief substance in solution in whey or serum. Milk-sugar approximates to dextrose in its action on polarised light. By boiling with sulphuric acid it is converted into dextrose and galactose. _Fat_ occurs in milk as suspended globules, and by churning may be made into butter. _The Proteids_ include casein, albumen, lactoprotein, and a small quantity of globulin. These are the nitrogenous bodies. _Mineral Matter._ The ash of milk, obtained by careful ignition of the solids, contains calcium, magnesium, potassium, sodium, phosphoric acid, sulphuric acid, chlorine, and iron, phosphoric acid and lime being present in the largest amounts. (1) _Lactic Acid Fermentation._ If milk is left undisturbed, it is well known that eventually it becomes sour. The casein is coagulated, and falls to the bottom of the vessel; the whey or serum rises to the top. In fact, a coagulation analogous to the clotting of blood has taken place. In addition to this, the whole has acquired an acid taste. Now, this double change is not due to any one of the constituents we have named above. It is, in short, a fermentation set up by a living ferment introduced from without. The constituent most affected by the ferment is the milk-sugar, which is broken down into lactic acid, carbonic acid gas, and other products.