68 
PEOFESSOE TYNDALL ON THE OPTICAL DEPOETMENT OF THE 
taining six tubes. On December 13, when the refuse was in a state of noisome putre- 
faction, infusions of whiting, turnip, beef, and mutton were placed in the other four 
tubes. They were boiled and abandoned to the action of the foul “ sewer-gas” emitted 
by their two putrid companions. On Christmas-day these four infusions were limpid. 
The end of the pipette was then dipped into one of the putrid tubes, and a quantity of 
matter, comparable in smallness to the pock-lymph held on the point of a lancet, was 
transferred to the turnip. Its clearness was not sensibly affected at the time ; but on 
the 26th it was turbid throughout. On the 27th a speck from the infected turnip was 
transferred to the whiting ; on the 28th disease had taken entire possession of the 
whiting. To the present hour the beef- and mutton-tubes remain as limpid as distilled 
water. Just as in the case of the living men and women in Edinburgh, no amount of 
fetid gas had the power of propagating the plague, as long as the organisms which con- 
stitute the true contagium did not gain access to the infusions. 
In the foregoing observations the tubes were arranged in the same horizontal plane ; 
but I also sought to obtain some notion of the vertical distribution of the germs in the 
air of the room. Two trays, each containing 100 tubes, were supported the one above 
the other in the same frame. The upper tray had all the air between it and the ceiling, 
a height of about 12 feet, from which the germs might descend upon it ; the lower tray 
was shaded by the upper, a space of only 6 inches existing between them. If the 
number of germs deposited in the tubes were determined by the air-space above, the 
upper tray would be the one most rapidly and thoroughly taken possession of. The 
reverse was the case. As regards the development of Bacterial life, the lower tray was 
from first to last in advance of its neighbour. It is not air-space, then, so much as still- 
ness, that determines the deposition of the germs. The air between the two trays being 
less disturbed than the general air of the room, the germs were less wafted about, and 
therefore fell in greater numbers into the tubes of the lower tray. We have here data 
which will enable us to form a rough notion of the lower limit of the number of germs 
contained in the room where the experiments were made. 
The floor of the room measured 20 feet by 15 feet; its area was therefore 43,200 
square inches, and every square inch would afford room for the section of one of our 
test-tubes. The height of the room is 180 inches; hence 30 layers of tubes 6 inches 
apart might be placed one above the other between the floor and ceiling. This would 
make 1,296,000 tubes. If only a single germ a day fell into each tube this would be 
the number of the germs. If the number deposited were one an hour, we should have 
thirty millions a day sown in the tubes. Probably the average time necessary for 
infection is very much less than an hour. At all events 30,000,000 of germs daily 
would be an exceedingly moderate estimate of the number falling into our thirty layers 
of tubes. This, moreover, would only be a fraction — probably a small fraction — of the 
germs really present in the air. In his Presidential Address to the British Association 
at Liverpool, Prof. Huxley ventured the statement that myriads of germs are floating in 
our atmosphere. Untrained experimenters and rash reasoners have ridiculed this state- 
