144 BACTERIA IN MILK. 
The Souring of Milk during Thunder Storms. The only natural 
agent that causes souring is the growth of microorganisms. 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 conditions 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 afternoon. The two phenomena 
occur together, not because the one causes the other, but because 
the same climatic conditions which produce the storm hasten the 
growth of bacteria. A similar warm spell will sour the milk just 
as quickly, even though no thunder storm appears. Whether this 
is the whole explanation may be doubtful, but it is clearly demon- 
strated that the thunder and lightning have nothing to do directly 
with the phenomenon. The souring of milk is always produced by 
bacteria. 
