out. Outside the body, bacteria may be killed by starvation,
by want of moisture, by being subjected to high temperature, or by the
action of certain chemical agents of which carbolic acid, the
perchloride and biniodide of mercury, and various chlorine preparations
are the most powerful.

#Immunity.#--Some persons are insusceptible to infection by certain
diseases, from which they are said to enjoy a _natural immunity_. In
many acute diseases one attack protects the patient, for a time at
least, from a second attack--_acquired immunity_.

_Phagocytosis._--In the production of immunity the leucocytes and
certain other cells play an important part in virtue of the power they
possess of ingesting bacteria and of destroying them by a process of
intra-cellular digestion. To this process Metchnikoff gave the name of
_phagocytosis_, and he recognised two forms of _phagocytes_: (1) the
_microphages_, which are the polymorpho-nuclear leucocytes of the blood;
and (2) the _macrophages_, which include the larger hyaline leucocytes,
endothelial cells, and connective-tissue corpuscles.

During the process of phagocytosis, the polymorpho-nuclear leucocytes in
the circulating blood increase greatly in numbers (_leucocytosis_), as
well as in their phagocytic action, and in the course of destroying the
bacteria they produce certain ferments which enter the blood serum.
These are known as _opsonins_ or _alexins_, and they act on the bacteria
by a process comparable to narcotisation, and render them an easy prey
for the phagocytes.

_Artificial or Passive Immunity._--A form of immunity can be induced by
the introduction of protective substances obtained from an animal which
has been actively immunised. The process by which passive immunity is
acquired depends upon the fact that as a result of the reaction between
the specific virus of a particular disease (the _antigen_) and the
tissues of the animal attacked, certain substances--_antibodies_--are
produced, which when transferred to the body of a susceptible animal
protect it against that disease. The most important of these antibodies
are the _antitoxins_. From the study of the processes by which immunity
is secured against the effects of bacterial action the serum and vaccine
methods of treating certain infective diseases have been evolved. The
_serum treatment_ is designed to furnish the patient with a sufficiency
of antibodies to neutralise the infection. The anti-diphtheritic and t