with the cherry-cask and beer-vat; we end with the body of man. There
are persons born with the power of interpreting natural facts, as
there are others smitten with everlasting incompetence in regard to
such interpretation. To the former class in an eminent degree
belonged the illustrious philosopher Robert Boyle, whose words in
relation to this subject have in them the forecast of prophecy. 'And
let me add,' writes Boyle in his 'Essay on the Pathological Part of
Physic,' 'that he that thoroughly understands the nature of ferments
and fermentations shall probably be much better able than he that
ignores them, to give a fair account of divers phenomena of several
diseases (as well fevers as others), which will perhaps be never
properly understood without an insight into the doctrine of
fermentations.'

Two hundred years have passed since these pregnant words were written,
and it is only in this our day that men are beginning to fully realise
their truth. In the domain of surgery the justice of Boyle's surmise
has been most strictly demonstrated. But we now pass the bounds of
surgery proper, and enter the domain of epidemic disease, including
those fevers so sagaciously referred to by Boyle. The most striking
analogy between a _contagium_ and a ferment is to be found in the power
of indefinite self-multiplication possessed and exercised by both. You
know the exquisitely truthful figures regarding leaven employed in the
New Testament. A particle hid in three measures of meal leavens it
all. A little leaven leaveneth the whole lump. In a similar manner,
a particle of _contagium_ spreads through the human body and may be so
multiplied as to strike down whole populations. Consider the effect
produced upon the system by a microscopic quantity of the virus of
smallpox. That virus is, to all intents and purposes, a seed. It is
sown as yeast is sown, it grows and multiplies as yeast grows and
multiplies, and it always reproduces itself. To Pasteur we are
indebted for a series of masterly researches, wherein he exposes the
looseness and general baselessness of prevalent notions regarding the
transmutation of one ferment into another. He guards himself against
saying it is impossible. The true investigator is sparing in the use
of this word, though the use of it is unsparingly ascribed to him;
but, as a matter of fact, Pasteur has never, been able to effect the
alleged transmutation, while he has been always able t