ual
experiments conducted immediately under my own eye, yet the evidence
I have adduced appears sufficient to establish it.

They who are not in the habit of conducting experiments may not be
aware of the coincidence of circumstances necessary for their being
managed so as to prove perfectly decisive; nor how often men engaged
in professional pursuits are liable to interruptions which disappoint
them almost at the instant of their being accomplished: however, I
feel no room for hesitation respecting the common origin of the
disease, being well convinced that it never appears among the cows
(except it can be traced to a cow introduced among the general herd
which has been previously infected, or to an infected servant),
unless they have been milked by some one who, at the same time, has
the care of a horse affected with diseased heels.

The spring of the year 1797, which I intended particularly to have
devoted to the completion of this investigation, proved, from its
dryness, remarkably adverse to my wishes; for it frequently happens,
while the farmers' horses are exposed to the cold rains which fall at
that season that their heels become diseased, and no Cow-pox then
appeared in the neighbourhood.

The active quality of the virus from the horses' heels is greatly
increased after it has acted on the nipples of the cow, as it rarely
happens that the horse affects his dresser with sores, and as rarely
that a milk-maid escapes the infection when she milks infected cows.
It is most active at the commencement of the disease, even before it
has acquired a pus-like appearance; indeed I am not confident whether
this property in the matter does not entirely cease as soon as it is
secreted in the form of pus. I am induced to think it does cease[1],
and that it is the thin darkish-looking fluid only, oozing from the
newly-formed cracks in the heels, similar to what sometimes appears
from erysipelatous blisters, which gives the disease. Nor am I
certain that the nipples of the cows are at all times in a state to
receive the infection. The appearance of the disease in the spring
and the early part of the summer, when they are disposed to be
affected with spontaneous eruptions so much more frequently than at
other seasons, induces me to think, that the virus from the horse
must be received upon them when they are in this state, in order to
produce effects: experiments, however, must determine these points.
But it is clear that whe