DIPHTHERIA. 305

of antitoxic serum seems to bear a distinct relation to
the age of the serum, fresh serums being more liable
to produce it than those which have been kept for a
month or two.

I have found that the c c keeping'' qualities of the se-
rums, when properly preserved, are of long duration.
Samples examined two years after having been exposed
for sale in the markets have been found unchanged.
The serums most prone to deteriorate seem to be those
of highest potency, but even here the good qualities are
unchanged for months.

Freezing is without effect and ordinary temperature-
changes are harmless to the serum. The antitoxic power
is destroyed at 72° C., the point at which the serum
coagulates.

The erythemata are probably in some way associated
with the globulicidal action of the blood. Keeping the
serum 'c until it is ripe'' lessens this effect. The serums
from different horses probably vary much in both their
irritant and globulicidal properties, so that antitoxins
prepared by mixing the serums from a number of horses
are probably preferable to those from single horses.

Dried serums are much less active than fresh ones.

For purposes of immunization smaller doses than those
used for treatment suffice. According to Biggs, 2 cubic
centimeters are sufficient to give complete protection.
The immunity that results from the injection is of a
month or six weeks' duration.

The transitory nature of this immunity is probably
dependent upon the fact that the antitoxin is slowly ex-
creted through the kidneys.

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