GENERAL CHARACTERS OF BACTERIA 21 
more moisture than most organisms. Some of them will hardly 
grow at all unless there is 30 per cent. of moisture in the material 
in which they are living, and even then the growth is slow. On 
the other hand, they flourish most luxuriantly in localities where 
the water is from 90 to 100 per cent. Hence, as materials dry, 
bacteria will cease to grow in them, and any substance that can 
be dried can be thoroughly protected from their action. This 
explains why dried fish and dried meat, fruits, dried milk, etc., 
will keep indefinitely. The drying, however, does not actually 
kill the bacteria, for although they do not grow when the water is 
extracted from them, they may remain alive for weeks, months, 
or even years. In other words, it is impossible to depend upon 
drying as a means of destroying bacteria, for, while many indi- 
viduals will fail to live, many others do not seem to be injured at 
all by the drying, and are capable of resuming life again as soon 
as they find moisture. 
Yeasts are much like bacteria in respect to need for water, 
and will not grow unless the water content is high. But other 
fungi with which we are concerned, the molds and mushrooms, 
can get along upon a smaller amount of water. Substances that 
are too dry to putrefy may be spoiled by molding; hence it is much 
more difficult to preserve certain kinds of partly dry food from 
molding than from decaying. Flour, in a flour barrel, may be- 
come musty from the development of molds, but it will hardly 
show signs of decay or putrefaction unless it becomes actually wet. 
Relation of Food.—Bacteria and the other fungi feed upon an 
immense variety of foods. A few of them are able to nourish 
themselves upon mineral matter, and some can gain their neces- 
sary carbon from the small amounts of carbon dioxide in the air, 
resembling, in this respect, the green plants that are engaged in 
building up starch and other organic compounds. This class of 
bacteria is of great theoretical interest and doubtless of much im- 
portance in nature. But the vast majority of microdrganisms 
are unable to use mineral foods alone, requiring, like animals, to 
