88 THE FLOATING-MATTER OP THE AIR. 



a plug of cotton-wool being permitted to supply its 

 place. The test-tubes contained infusions of hay, tur- 

 nip, 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 pre- 

 cisely 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 propagating-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 

 became 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. 



§ 1 9. Experiments with Calcined Air. 



Six years ago^ I showed that the floating matter of 

 London air could be 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. 5. A glass shade, s, is placed upon a 

 slab of wood mounted on a tripod. Through the slab 

 passes three large test-tubes nearly filled with the in- 

 fusion to be examined. A platinum spiral, p, unites 

 ' Proo. Koy. Inst. vol. vi. pp. 4 ancl 5. 



