fifth day. 9 "

"On none of the five nights did a single conidium alight on the slides.
This seemed to me to prove that during the day the conidia, through the
dryness of the atmosphere and the shaking of the leaves, became detatched
and wafted by the air; while during the night the moisture (in the form
of dew, and on one occasion of a slight and gently falling shower)
prevented the drying of the conidia, and thus rendered them less easy of
detachment.

"I determined the nature of the conidia (1) by comparing them with
authentic conidia directly removed from diseased plants; (2) by there
being attached to some of them portions of the characteristic
conidiaphores; and (3) by cultivating them in a moist chamber, the result
of which was, that five conidia, not having been immersed in the
glycerine, retained their vitality, which they showed by bursting and
producing zoospores in the manner characteristic of _Peronospora
infestans_."

INFLUENZA.

Let us look at another disease by the light of recent knowledge, viz.,
the epidemic influenza, concerning which I remember hearing much talk, as
a child, in 1847-48. There has been no epidemic of this disease in the
British Isles since 1847, but we may judge of its serious nature from the
computation of Peacock that in London alone 250,000 persons were stricken
down with it in the space of a few days. It is characteristic of this
disease that it invades a whole city, or even a whole country, at "one
fell swoop," resembling in its sudden onset and its extent the potato
disease which we have been considering.

The mode of its spreading forbids us to attribute it, at least in any
material degree, although it may be partially so, to contagion in the
ordinary sense, i.e., contagion passing from person to person along the
lines of human intercourse. It forbids us also to look at community of
water supply or food, or the peculiarities of soil, for the source of the
disease virus. We look, naturally, to some atmospheric condition for the
explanation. That the atmosphere is the source of the virus is made more
likely from the fact that the disease has broken out on board ship in a
remarkable way. In 1782, there was an epidemic, and on May 2 in that
year, says Sir Thomas Watson--

"Admiral Kempenfelt sailed from Spithead with a squadron, of which the
Goliah was one. The crew of that vessel were attacked with influenza on
May 29, and the rest were at different times af