e doctor was taking away after
the death of the victim, and, while doubtless the size of the bottle and
the amount of the membrane has been magnified by the lapse of years, it
still remains to him as a terrible visitation and an inevitable cause of
death.

_Cause of the disease._

The immediate cause of diphtheria has been known only within recent
years. Sewer air was for a long time thought to be responsible, and
overcrowding or congestion in tenements was believed to be a fruitful
source of the disease. Some years ago, when diphtheria had been epidemic
in one of the state institutions and when experts had been called in to
suppress the disease, the elaborate reports which they made dwelt on the
quality of the drinking water and on the method of disposal of the
sewage as if those factors would account for the disease. About
twenty-five years ago, it was shown definitely that the disease was due
to certain bacteria, and that while the membrane in the throat was the
result of the rapid development of these bacteria, yet the mortality
from the disease was not due to the suppression of the act of breathing,
but to the development of a poison by the bacteria which went into the
circulation of the body and produced death, just as any poison, as
strychnine, for example, would do.

When once this fact was accepted, namely, that the disease was dangerous
because of the poisons involved, scientists undertook to find a way to
neutralize these poisons, and it was soon discovered that such
neutralizing substances could be grown in the blood of guinea pigs. It
was found that if a small dose of diphtherial toxin was injected into a
guinea pig,--a dose small enough so that the guinea pig would
recover,--it could then be given a larger dose from which it would also
recover. This process might be repeated, until at the end of several
weeks it could be given a dose the size of which would have been
sufficient to have killed it almost instantly at the beginning, and
which it could take and enjoy at the end of the series. The point was
that evidently, as with smallpox, successive inoculations resulted in
the formation in the body of some substance or agent capable of
neutralizing the poisons of the disease, subsequently formed. The guinea
pig is so small that the amount of restraining substance available made
it desirable to find a larger animal, and the horse, equally susceptible
to the disease with the guinea pig, was selected as the