ATMOSPHERE IN RELATION TO PUTREFACTION AND INFECTION. 
45 
be crowded with particles — not hypothetical, not potential, but actual and myriadfold 
in number — showing the microscopist that there is a world beyond even his range. 
In §§ 6 and 8 experiments on the infection of clear infusions by others containing visible 
Bacteria are referred to. But for the infection to be sure it is not necessary that the 
Bacteria should be visible. Over and over again I have repeated the experiments of Dr. 
Burdon Sanderson on the infective power of ordinary distilled water, in which the 
microscope fails to reveal a Bacterium. The water, for example, furnished to the Royal- 
Institution laboratory by Messrs. Hopkin and Williams is sensibly as infectious as an 
infusion swarming with Bacteria. 
Perhaps the severest experiment of this kind ever made was one executed by Dr. 
Sanderson with water prepared by myself. In 1871 I sought anxiously and assiduously 
for water free from suspended particles. The liquid was obtained in various degrees of 
purity, but never entirely pure. Knowing the wonderful power of extrusion, as regards 
foreign matter, brought into play by water 
in crystallizing, the thought occurred to me 
of examining the liquid derived from the 
fusion of the most transparent ice. Mr. Cot- 
trell, at my request, arranged the following 
apparatus for me : — Through the plate of an 
air-pump (fig. 5) passed air-tight the shank of a 
large funnel. A small glass bulb, B, furnished 
with a glass stopcock, was attached to the 
shank of the funnel below. Prior to being 
put together all parts of the apparatus had 
been scrupulously cleansed. In the funnel 
was placed a block of ice, I, selected for its 
transparency, having a volume of 1000 cubic 
inches or thereabouts, and over the ice was 
placed an air-tight receiver. Several times 
in succession the air was removed from this 
receiver, its place on each occasion being 
taken by other air carefully filtered through 
cotton-wool. The transparent ice was thus 
surrounded by moteless air. 
The ice was now permitted to melt ; its water trickled into the small glass bulb below, 
which was filled and emptied a great number of times. From the very heart of the 
block of ice the water was finally taken and subjected to the scrutiny of the concentrated 
beam. It proved to be the purest liquid I had ever seen — probably the purest 
human eye had ever seen ; but still it contained myriads of ultra-microscopic particles. 
The track of the beam through it was of the most delicate blue, the blue light being 
perfectly polarized. It could be wholly quenched by a Nicol’s prism, the beam then 
h 2 
