cinated, is the solid
particles and not the fluid. By experiments of the most ingenious kind,
the solid parts have been separated from the fluid parts, and it has
then been discovered that you may vaccinate a child as much as you like
with the fluid parts, but no effect takes place, though an excessively
small portion of the solid particles, the most minute that can be
separated, is amply sufficient to give rise to all the phenomena of
the cow pock, by a process which we can compare to nothing but the
transmission of fermentation from one vessel into another, by the
transport to the one of the torula particles which exist in the other.
And it has been shown to be true of some of the most destructive
diseases which infect animals, such diseases as the sheep pox, such
diseases as that most terrible and destructive disorder of horses,
glanders, that in these, also, the active power is the living solid
particle, and that the inert part is the fluid. However, do not suppose
that I am pushing the analogy too far. I do not mean to say that the
active, solid parts in these diseased matters are of the same nature as
living yeast plants; but, so far as it goes, there is a most surprising
analogy between the two; and the value of the analogy is this, that by
following it out we may some time or other come to understand how these
diseases are propagated, just as we understand, now, about fermentation;
and that, in this way, some of the greatest scourges which afflict the
human race may be, if not prevented, at least largely alleviated.

This is the conclusion of the statements which I wished to put before
you. You see we have not been able to have any accessories. If you will
come in such numbers to hear a lecture of this kind, all I can say is,
that diagrams cannot be made big enough for you, and that it is not
possible to show any experiments illustrative of a lecture on such a
subject as I have to deal with. Of course my friends the chemists and
physicists are very much better off, because they can not only show you
experiments, but you can smell them and hear them! But in my case such
aids are not attainable, and therefore I have taken a simple subject and
have dealt with it in such a way that I hope you all understand it,
at least so far as I have been able to put it before you in words; and
having once apprehended such of the ideas and simple facts of the case
as it was possible to put before you, you can see for yourselves th