FERMENTATION 283 



ment more conclusive nor an inference safer than this 

 one. 



Supposing the powder to be light enough to float in 

 the air, and that you are enabled to see it there just as 

 plainly as you saw the heavier powder in the palm of 

 your hand. If the dust sown by the air instead of by the 

 hand produce a definite living crop, with the same logical 

 rigor you would conclude that the germs of this crop 

 must be mixed with the dust. To take an illustration: 

 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 PenicilUum. 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, 



