"It is some years since I proposed to combat intestinal putrefaction and its injurious consequences by means of lactic ferments. I thought the acidity produced by such microbes would be much more effective in preventing the germination of putrefying microbes than the small quantity of acids produced by _Bacillus coli_. On the other hand, I had no illusion as to the difficulty sure to be encountered in any effort to introduce lactic microbes into the intestinal flora which has been preoccupied by a multitude of other microbes. To make surer of the result, I chose the lactic microbe, which is the strongest as an acid producer. It is found in the _yahourt_ (yoghourt), which originates in Bulgaria. The same bacillus has also been isolated from the _leben_ of Egypt; and it is now proved that it is found in the curdled milk of the whole Balkan peninsula, and even in the Don region of Russia."[41] It is a short step from considerations like these to the adoption of the _Bacillus bulgaricus_ as the most potent of the various lactic organisms which have been examined, and which is likely to play such an important rôle in the destiny of the human race. The _Bacillus bulgaricus_ may claim to be the Bacillus of Long Life. CHAPTER III THE CHEMISTRY OF MILK _The Composition of Milk._—Like all other organic substances, or those built up in connection with the life processes of plants and animals, milk is of complex composition. It is also very liable to change—every one is acquainted with its tendency to "go bad." This instability is more or less inherent in all highly organised chemical compounds, and, indeed, it seems to be necessary that the materials used in growth and nutrition should be very plastic in a chemical sense, in order, _e.g._, that the constituents, say of a plant, may easily be transformed into the substances of the body of the animal which feeds on it. The perishable nature of milk—the food of young and growing animals—is therefore essential, so that it may be changed easily into the blood, bone, muscle, etc., so abundantly required in the early stages of existence. Milk is a complete food, and, therefore, naturally it is not a simple chemical compound, but a mechanical mixture of a number of substances. The present state of chemical knowledge on the subject does not permit of its composition being given in detail, but for practical purposes, such as those of measuring its purity and food value, this is not necessary. A proximate analysis, in which, at least, some of the ingredients are lumped together, is sufficient, and has been adopted everywhere by analysts. On this basis the average composition of cows' milk may be stated as follows: Per cent. Water 87.50 Fat 3.50 Casein and albumen 3.65 Milk sugar 4.60 Ash 0.75 ──── 100.00 ──────