trustworthy illustration. In
1850, two distinguished French observers, MM. Davainne and Rayer,
noticed in the blood, of animals which had died of the virulent
disease called splenic fever, small microscopic organisms resembling
transparent rods, but neither of them at that time attached any
significance to the observation. In 1861, Pasteur published a memoir
on the fermentation of butyric acid, wherein he described the organism
which provoked it; and after reading this memoir it occurred to
Davainne that splenic fever might be a case of fermentation set up
within the animal body, by the organisms which had been observed by
him and Rayer. This idea has been placed beyond all doubt by
subsequent research.

Observations of the highest importance have also been made on splenic
fever by Pollender and Brauell. Two years ago, Dr. Burdon Sanderson
gave us a very clear account of what was known up to that time of this
disorder. With regard to the permanence of the _contagium_, it had been
proved to hang for years about localities where it had once prevailed;
and this seemed to show that the rod-like organisms could not
constitute the _contagium_, because their infective power was found to
vanish in a few weeks. But other facts established an intimate
connection between the organisms and the disease, so that a review of
all the facts caused Dr. Sanderson to conclude that the _contagium_
existed in two distinct forms: the one 'fugitive' and visible as
transparent rods; the other permanent but 'latent,' and not yet
brought within the grasp of the microscope.

At the time that Dr. Sanderson was writing this report, a young German
physician, named Koch, [Footnote: This, I believe, was the first
reference to the researches of Koch made in this country. 1879.]
occupied with the duties of his profession in an obscure country
district, was already at work, applying, during his spare time,
various original and ingenious devices to the investigation of splenic
fever. He studied the habits of the rod-like organisms, and found the
aqueous humour an ox's eye to be particularly suitable for their
nutria. With a drop of the aqueous humour he mixed tiniest speck of a
liquid containing the rods, placed the drop under his microscope,
warmed it suitably, and observed the subsequent action. During the
first two hours hardly any change was noticeable; but at the end of
this time the rods began to lengthen, and the action was so rapid that