62 
PEOFESSOE TYNDALL ON THE OPTICAL DEPOETMENT OF THE 
germs in the air, observations outside of London would, I thought, be interesting. Accord- 
ingly, on the 27th of October, a tube containing an infusion of beef was placed in the hands 
of Mr. Darwin, who had the kindness to set it in his study at Down and observe its changes. 
In three days it became cloudy and peopled with Bacteria. The same result was obtained 
in the open air. Mr. Francis Darwin was good enough to expose an infusion for me in his 
father’s orchard : the weather was cold, and the progress, therefore, slow ; but the tube 
which had been exposed on the 2nd of November was cloudy and full of Bacteria on 
the 9th. In Sir John Lubbock’s study a similar result was obtained. From Sherwood, 
near Tunbridge Wells, infusions of fowl and wild duck were returned to me by 
Mr. Siemens thickly turbid and crowded with Bacteria. From Pembroke Lodge, Kich- 
mond Park, Mr. Kollo Eussell returned tubes of turnip, beef, and mutton swarming 
with life. An infusion of beef exposed at Heathfield Park, Sussex, for a week was returned 
to me by Miss Hamilton muddy and filled with Bacteria. From Greenwich Hospital 
Mr. Hirst sent me tubes of beef-, mutton-, and turnip-infusion filled with vigorous 
Bacteria. Dr. Hooker was good enough to take charge of three sets of tubes at Kew. 
One set was placed in the conservatory, with a temperature of 45° to 50°; one in his 
own study, with a temperature of 54° to 60°; a third set was placed in the orchid-house 
(the hottest in the gardens), with a temperature of 62° to 75°. 
The tubes were opened on the 4th of December, all of them being then clear. In 
the orchid-house the turnip became cloudy on the 7th, the two others on the 8th, after 
which the opacity rapidly increased. In the study all remained clear until the 9th, 
when the turnip began to cloud. On the 11th the beef was still clear, while the mutton 
had given way. On the 13th all of them had yielded. In the conservatory the turnip 
began to cloudonthelOth; the others followed much in the same order as in the other cases. 
The influence of temperature seems well shown by these observations. Three days 
sufficed to cloud the turnip in the orchid-house, five days in the study, and six days in 
the conservatory. The mutton in the study gathered over it a thick blanket of Beni- 
cillium. On the 13th it had assumed a light brown colour, “ as if by a faint admixture 
of clay;” but the infusion became transparent. The “ clay ” here was the slime of dor- 
mant or dead Bacteria , the cause of their quiescence being the blanket of Penicillium. 
I found no active life in this tube, while all the others swarmed with Bacteria. From 
the Crystal Palace at Sydenham Mr. Price sent me tubes of mutton, beef, and turnip 
charged with Bacteria. The temperature was low at night, the development of life being 
thereby considerably retarded. 
Thus everywhere it has been tested the atmosphere has been found charged with 
the germs of Bacteria. 
I wished, however, to obtain clearer and more definite insight as to the diffusion of 
atmospheric germs. Supposing a large tray to be filled with a suitable organic infusion 
and exposed to the air. Into it the germs would drop ; and could the resulting organisms 
be confined to the locality where the germs fell, we should have the floating life of the 
