FERMENTATION. 545 



the spores of the little plant Penicillium glaucum, to 

 which I have already referred, are light enough to float 

 in the air. A cut apple, a pear, a tomato, a slice of vege- 

 table marrow, or, as already mentioned, an old moist boot, 

 a dish of paste, or a pot of jam, constitutes a proper soil 

 for the Penicillium. Now, if it could be proved that the 

 dust of the air when sown in this soil produces this plant, 

 while, wanting the dust, neither the air, nor the soil, nor 

 both together can produce it, it would be obviously just 

 as certain in this case that the floating dust contains the 

 germs of Penicillium as that the powders sown in your 

 garden contained the germs of the plants which sprung 

 from them. 



But how is the floating dust to be rendered visible? In 

 this way. Build a little chamber and provide it with a 

 door, windows, and window-shutters. Let an aperture be 

 made in one of the shutters through which a sunbeam can 

 pass. Close the door and windows so that no light shall 

 enter save through the hole in the shutter. The track of 

 the sunbeam is at first perfectly plain and vivid in the 

 air of the room. If all disturbance of the air of the 

 chamber be avoided, the luminous track will become 

 fainter and fainter, until at last it disappears absolutely, 

 and no trace of the beam is to be seen. What rendered the 

 beam visible at first? The floating dust of the air, which, 

 thus illuminated and observed, is as palpable to sense as 

 dust or powder placed on the palm of the hand. In the 

 still air the dust gradually sinks to the floor or sticks to 

 the walls and ceiling, until finally, by this self-cleansing 

 process, the air is entirely freed from mechanically sus- 

 pended matter. 



Thus far, I think, we have made our footing sure. Let 

 us proceed. Chop up a beefsteak and allow it to remain 

 for two or three hours just covered with warm water; you 

 thus extract the juice of the beef in a concentrated form. 

 By properly boiling the liquid and filtering it, you can 

 obtain from it a perfectly transparent beef-tea. Expose a 

 number of vessels containing this tea to the moteless air of 

 your chamber; and expose a number of vessels containing 

 precisely the same liquid to the dust-laden air. In three 

 days every one of the latter stinks, and examined with the 

 microscope every one of them is found swarming with the 

 bacteria of putrefaction. After three months, or three 



