made a wide and general inquiry into the
causes which tend to shorten life, and makes out a strong case in
support of the view that in many cases this is the result of what is
called auto-intoxication or self-poisoning. In man and in the mammalia
generally, the colon or large intestine is very largely developed; this
organ is not of much value in the digestion of food, and seems to be
chiefly a receptacle for waste material; it is, as a rule, extremely
rich in bacterial flora, which produce putrefactive changes in the waste
material. As a result various poisonous principles are evolved, and
these find their way into the blood, accompanied frequently, there is
every reason to believe, by the injurious bacteria themselves. In this
way many diseases and ailments are produced which shorten life. The
inquiry then naturally turns to what is the best way of dealing with
this state of matters. It is known that the bacteria referred to
flourish best in alkaline or at least non-acid surroundings, and it is
known that these conditions very frequently exist in the large
intestine. Acids are the best antiseptics; they have been used from time
immemorial as preservatives; pickles are preserved in vinegar or acetic
acid, and when milk is allowed to sour under proper conditions, the
germs of putrefaction are destroyed or their activity inhibited, and it
keeps a considerable time. Doubtless, in hot countries, it was this
property of lactic acid which first led to milk being artificially
soured with a view to its preservation as an article of food. So
powerful is lactic acid in this respect that it is a custom in some
countries to preserve meat by placing it in soured milk.

How can acids be applied so as to control the bacterial flora of the
large intestine? Not in the ordinary way, because, when administered
through the mouth, they are used up long before they can penetrate to
the colon. The brilliant idea occurred to Professor Metchnikoff, of
administering acid-producing germs which might work their way through
the digestive system, and, reaching the large intestine, produce the
acid required. After much experimenting the bacillus of Massol,
_Bacillus bulgaricus_, was adopted as the most suitable. The Bulgarian
bacillus is an extremely vigorous one, multiplying with great rapidity,
and persisting in conditions that would be inimical to other microbes.
The growth and development of bacteria is interfered with by the
products of their o