66 
PROEESSOR TYNDALL ON THE OPTICAL DEPORTMENT OE THE 
bottom as a sediment. The growth of mould and its effect on the Bacteria are very 
capricious. The turnip-infusion, after developing in the first instance its myriadfold 
Bacterial life, frequently rapidly contracts mould, which stifles the Bacteria and clears 
the liquid all the way between the sediment and the scum. Of two tubes placed beside 
each other, one will be taken possession of by Bacteria , which successfully fight the 
mould and keep the surface perfectly clean ; while another will allow the mould a 
footing, the apparent destruction of the Bacteria being the consequence. This I have 
proved to be the case with all infusions, fish, flesh, fowl, and vegetable. At the present 
moment, for example, of three tubes containing an infusion of sole, placed close together 
in a row, the two outside ones are covered by a thick tough blanket of mould, while the 
central one has not a single speck upon its surface. The Bacteria which manufacture 
a green pigment appear to be uniformly victorious in their fight with the Penicillium. 
These observations enable us, I think, to draw some interesting conclusions. From 
the irregular manner in which the tubes are attacked we may infer that, as regards 
quantity, the distribution of the germs in the air is not uniform. A single tube will 
sometimes be a day or more in advance of its neighbours. The singling out, more- 
over, of one tube of the hundred by the particular Bacteria that develop a green 
pigment, and other cases just adverted to, shows that, as regards quality , the distribution 
is not uniform. This has been further illustrated by the following observations : — Of 
five and twenty tubes of different animal infusions exposed in groups of five, in the 
middle of November, and all swarming with Bacterial life, five were green. They 
were distributed as follows: — Beef 2, herring 1, haddock 1, fowl 1, wild duck 0. 
The same absence of uniformity was manifested in the struggle for existence between 
the Bacteria and the Penicillium. In some tubes the former were triumphant; in 
other tubes of the same infusion the latter was triumphant. It would also seem that a 
want of uniformity as regards vital vigour prevailed. With the selfsame infusion the 
motions of the Bacteria in some tubes were exceedingly languid ; while in other tubes 
the motions resembled a rain of projectiles, being so rapid and violent as to be followed 
with difficulty by the eye. Reflecting on the whole of this, I conclude that the germs 
float through the atmosphere in groups or clouds, and that now and then a cloud speci- 
fically different from the prevalent ones is wafted through the air. The touching of 
a nutritive fluid by a Bacterial cloud would naturally have a different effect from the 
touching of it by the interspace between two clouds. But, as in the case of a mottled 
sky, the various portions of the landscape are successively visited by shade, so, in the 
long run, are the various tubes of our tray touched by the Bacterial clouds, the final 
fertilization or infection of them all being the consequence*. 
* In hospital practice the opening of a wound during the passage of a Bacterial cloud would have an effect 
very different from the opening of it in the interspace between two clouds. Certain caprices in the behaviour 
of dressed wounds may possibly be accounted for in this way. 
Under the heading “ Nothing New under the Sun,” Prof. Huxley has lately sent me the following remarkable 
