50 
PEOFESSOE TYNDALL ON THE OPTICAL DEPOETMENT OF THE 
with cement, in which, while it was hot and soft, a heated “ propagating-glass,” 
resembling a huge bell-jar, was imbedded. The air within the glass was pumped out 
several times, air filtered carefully through a plug of cotton-wool being permitted to 
supply its place. The test-tubes contained infusions of hay, turnip, beef, and mutton, 
three of each, twelve in all. For two months they remained as clear and cloudless as 
they were upon the day of their introduction, while twelve similar tubes, prepared at 
the same time, in precisely the same way, and hung on to the slab of wood outside the 
propagating-glass, were, in less than a week, clogged with mycelium, mould, and 
Bacteria. 
One of the protected tubes was accidentally broken, and though its aperture was rapidly 
plugged with cotton-wool, some common air must, at the time, have entered the propa- 
gating-glass. Evaporation from the infusions went on ; the vapour was condensed by the 
glass above, trickled down its interior surface, carrying with it, in part, such matter as 
had attached itself to that surface. A kind of pool was thus formed upon the cement 
below. This, after an interval of three months, is now spotted with disks of Penicillium , 
by the spores of which one or two of the infusions have been recently invaded, the 
production of very beautiful mycelium-tufts being the consequence. 
§ 19. Experiments with Calcined Air. 
Six years ago * I showed that the floating matter of London air could be completely 
removed by permitting a platinum wire heated to whiteness to act upon it for a sufficient 
time. I availed myself of this mode of calcining the air on the present occasion. The 
apparatus employed is shown in fig. 7. A glass shade, S, is 
placed upon a slab of wood mounted on a tripod, and through 
which passes three large test-tubes nearly filled with the 
infusion to be examined. A platinum spiral,^), unites the ends 
of two upright copper wires, which pass through the stand and 
are seen coiled outside it. The shade is surrounded by a tin 
collar, with a space of about half an inch all round between it 
and the shade. This space is filled with cotton-wool firmly 
packed. Connecting the wires with a battery of fifteen cells, 
the spiral^) was raised to whiteness, and was permitted to con- 
tinue so for five minutes. Experiments previously executed 
had shown that this sufficed for the entire removal of the 
floating matter. When the spiral was heated, a portion of the 
expanded air was driven through the cotton-wool packing below ; and when the current 
was interrupted, this air, returning into the shade, was prevented by the cotton-wool 
from carrying any floating matter with it. 
The first three substances brought into contact with air calcined in this way were 
damson-juice, pear-juice, and infusion of yeast. They were boiled for five minutes, and 
* Proc. Eoy. Inst. vol. vi. pp. 4 & 5. 
Fi{?. 7. 
