urposes. The so-called chemical
preservatives used to "keep" milk depend for their effect on the
inhibition of bacterial growth. With a substance so violently toxic as
formaldehyde (known as formalin, freezene) antiseptic doses are likely
to be exceeded. In this country most states prohibit the use of these
substances in milk. Their only function in the dairy should be to check
fermentative or putrefactive processes outside of milk and so keep the
air free from taints.

~Products of growth.~ All bacteria in their development form certain more
or less characteristic by-products. With most dairy bacteria, these
products are formed from the decomposition of the medium in which the
bacteria may happen to live. Such changes are known, collectively, as
fermentations, and are characterised by the production of a large amount
of by-products, as a result of the development of a relatively small
amount of cell-life. The souring of milk, the formation of butyric acid,
the making of vinegar from cider, are all examples of fermentative
changes.

With many bacteria, especially those that affect proteid matter,
foul-smelling gases are formed. These are known as putrefactive changes.
All organic matter, under the action of various organisms, sooner or
later undergoes decay, and in different stages of these processes,
acids, alkalies, gases and numerous other products are formed. Many of
these changes in organic matter occur only when such material is brought
in direct contact with the living bacterial cell.

In other instances, soluble, non-vital ferments known as _enzyms_ are
produced by the living cell, which are able to act on organic matter, in
a medium free from live cells, or under conditions where the activity of
the cell is wholly suspended. These enzyms are not confined to bacteria
but are found throughout the animal and plant world, especially in those
processes that are concerned in digestion. Among the better known of
these non-vital ferments are rennet, the milk-curdling enzym; diastase
or ptyalin of the saliva, the starch-converting enzym; pepsin and
trypsin, the digestive ferments of the animal body.

Enzyms of these types are frequently found among the bacteria and yeasts
and it is by virtue of this characteristic that these organisms are
able to break down such enormous quantities of organic matter. Most of
these enzyms react toward heat, cold and chemical poisons in a manner
quite similar to the living cells. In o