somewhat variable, probably constitute a single species, and are identical with the Bacillus acidi lactici (Fig. 19).  This species, found common in the dairies of Europe, according to recent investigations occurs in this country as well.  We may say, then, that while there are many species of bacteria infesting the dairy which can sour the milk, there is one which is more common and more universally found than others, and this is the ordinary cause of milk souring.

When we study more carefully the effect upon the milk of the different species of bacteria found in the dairy, we find that there is a great variety of changes which they produce when they are allowed to grow in milk.  The dairyman experiences many troubles with his milk.  It sometimes curdles without becoming acid.  Sometimes it becomes bitter, or acquires an unpleasant “tainted” taste, or, again, a “soapy” taste.  Occasionally a dairyman finds his milk becoming slimy, instead of souring and curdling in the normal fashion.  At such times, after a number of hours, the milk becomes so slimy that it can be drawn into long threads.  Such an infection proves very troublesome, for many a time it persists in spite of all attempts made to remedy it.  Again, in other cases the milk will turn blue, acquiring about the time it becomes sour a beautiful sky-blue colour.  Or it may become red, or occasionally yellow.  All of these troubles the dairyman owes to the presence in his milk of unusual species of bacteria which grow there abundantly.

Bacteriologists have been able to make out satisfactorily the connection of all these infections with different species of the bacteria.  A large number of species have been found to curdle milk without rendering it acid, several render it bitter, and a number produce a “tainted” and one a “soapy” taste.  A score or more have been found which have the power of rendering the milk slimy.  Two different species at least have the power of turning the milk to sky-blue colour; two or three produce red pigments (Fig. 20), and one or two have been found which produce a yellow colour.  In short, it has been determined beyond question that all these infections, which are more or less troublesome to dairymen, are due to the growth of unusual bacteria in the milk.

These various infections are all troublesome, and indeed it may be said that, so far as concerns the milk producer and the milk consumer, bacteria are from beginning to end a source of trouble.  It is the desire of the milk producer to avoid them as far as possible—­a desire which is shared also by everyone who has anything to do with milk as milk.  Having recognised that the various troubles, which occasionally occur even in the better class of dairies, are due to bacteria, the dairyman is, at least in a measure, prepared to avoid them.  The avoiding of these troubles is moderately easy as soon as dairymen recognise the source from which the infectious organisms come, and also the fact that low temperatures