while this is a decidedly extreme number, milk that is a day old will almost always contain many millions in each cubic inch, the number depending upon the age of the milk and its temperature.  During this growth the bacteria have, of course, not been without their effect.  Recognising as we do that bacteria are agents for chemical change, we are prepared to see the milk undergoing some modifications during this rapid multiplication of bacteria.  The changes which these bacteria produce in the milk and its products are numerous, and decidedly affect its value.  They are both advantageous and disadvantageous to the dairyman.  They are nuisances so far as concerns the milk producer, but allies of the butter and cheese maker.

The effect of bacteria on milk.

The first and most universal change effected in milk is its souring.  So universal is this phenomenon that it is generally regarded as an inevitable change which can not be avoided, and, as already pointed out, has in the past been regarded as a normal property of milk.  To-day, however, the phenomenon is well understood.  It is due to the action of certain of the milk bacteria upon the milk sugar which converts it into lactic acid, and this acid gives the sour taste and curdles the milk.  After this acid is produced in small quantity its presence proves deleterious to the growth of the bacteria, and further bacterial growth is checked.  After souring, therefore, the milk for some time does not ordinarily undergo any further changes.

Milk souring has been commonly regarded as a single phenomenon, alike in all cases.  When it was first studied by bacteriologists it was thought to be due in all cases to a single species of micro-organism which was discovered to be commonly present and named Bacillus acidi lactici (Fig. 19).  This bacterium has certainly the power of souring milk rapidly, and is found to be very common in dairies in Europe.  As soon as bacteriologists turned their attention more closely to the subject it was found that the spontaneous souring of milk was not always caused by the same species of bacterium.  Instead of finding this Bacillus acidi lactici always present, they found that quite a number of different species of bacteria have the power of souring milk, and are found in different specimens of soured milk.  The number of species of bacteria which have been found to sour milk has increased until something over a hundred are known to have this power.  These different species do not affect the milk in the same way.  All produce some acid, but they differ in the kind and the amount of acid, and especially in the other changes which are effected at the same time that the milk is soured, so that the resulting soured milk is quite variable.  In spite of this variety, however, the most recent work tends to show that the majority of cases of spontaneous souring of milk are produced by bacteria which, though