156 BACTERIA IN MILK 
state of partial solution, but if the milk is made sufficiently acid 
the casein can no longer remain in solution and is precipitated. 
The precipitation of casein is the curdling of milk, and it occurs 
when we add to it any kind of acid. 
The Souring of Milk during Thunder Storms —The only natural 
agent that causes souring is the growth ofmicroérganisms. There 
is, however, a wide-spread belief that thunder storms will sour and 
curdle milk. This belief rests upon a mistaken interpretation of 
observed facts. It is certainly true that milk is frequently found 
sour after a thunder storm, and the natural interpretation is that 
the electricity of the storm has produced the souring. A careful 
study of the phenomenon has shown that this inference is incorrect. 
Electricity, in the form of a current or electric sparks, has no power 
to sour milk; and, further, if milk is kept properly cooled, the 
thunder storm has no effect upon it. Moreover, if milk has been 
deprived of bacteria, it will keep indefinitely, remaining sweet in 
spite of thunder storms. In short, all evidence shows that the 
thunder storm has no power of souring milk, unless bacteria are 
present to produce the lactic acid, and that thunder and lightning 
have no direct effect upon the souring of milk. 
What, then, brings about the frequent souring of milk during 
thunder storms and the wide-spread belief that thunder is the 
cause? The answer seems to be the simple one, that the same 
agencies which produce the thunder storm cause a rapid growth of 
bacteria. The thunder storm is brought on by climatic conditions, 
dependent chiefly upon the temperature, and these same condi- 
tions are just those that stimulate bacterial growth. It will thus 
happen that the same sort of warm weather which produces the 
thunder storm also hastens the growth of bacteria in milk if not 
kept artificially cooled with ice. It will frequently happen, as a 
result, that the milk will be ready to show signs of souring at the 
same time that the thunder storm appears, frequently in the after- 
noon. The two phenomena occur together, not because the one 
causes the other, but because the same climatic conditions which 
