s comparatively loose, and
belongs to the class of reversible actions, being comparable in fact with
the union of a weak acid and base. If such were the condition there would
always be a certain amount both of free toxin and of free antitoxin in the
mixture, and in this case also considerably more than a dose of toxin would
have to be added to a "neutral mixture" before the amount of free toxin was
increased by a dose, that is, before the mixture became lethal. It may be
stated that while in certain instances the union of toxin and antitoxin may
be reversible, all the facts established cannot be explained on this simple
hypothesis of reversible action. Still another view, advocated by Bordet,
is that the union of toxin and antitoxin is rather of physical than of
strictly chemical nature, and represents an interaction of colloidal
substances, a sort of molecular deposition by which the smaller toxin
molecule becomes entangled in the larger molecule of antitoxin. Sufficient
has been said to show that the subject is one of great intricacy, and no
simple statement with regard to it is as yet possible. We are probably safe
in saying, however, that the molecules of a toxin are not identical but
vary in the degree of their combining affinities, and also in their toxic
action, and that, while in some cases the combination of anti-substances
has been shown to be reversible, we are far from being able to say that
this is a general law.

[Sidenote: Formation of antitoxin.]

The origin of antitoxin is of course merely a part of the general question
regarding the production of anti-substances in general, as these all
combine in the same way with their homologous substances and have the same
character of specificity. As, however, most of the work has been done with
regard to antitoxin production we may consider here the theoretical aspect
of the subject. There are three chief possibilities: (a) that the antitoxin
is a modification of the toxin; (b) that it is a substance normally
present, but produced in excess under stimulation of the toxin; (c) that it
is an entirely new product. The first of these, which would imply a process
of a very remarkable nature, is disproved by what is observed after
bleeding an animal whose blood contains antitoxin. In such a case it has
been shown that, without the introduction of fresh toxin, new antitoxin
appears, and therefore must be produced by the living tissues. The second
theory is the more prob