FERMENTATION. 269 



size of impalpable dust particles ; assuming that you 

 treat them all in the same way, and that from every 

 one of them in a few days you obtain a definite crop 

 it may be clover, it may be mustard, it may be mignon- 

 ette, it may be a plant more minute than any of these, 

 the smallness of the particles, or of the plants that spring 

 from them, does not affect the validity of the conclu- 

 sion. Without a shadow of misgiving you would con- 

 clude that the powder must have contained the seeds 

 or germs of the life observed. There is not in the 

 range of physical science, an experiment more conclu- 

 sive 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 rigour 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 

 enoiugh to float in the air. A cut apple, a pear, a 

 tomato, a slice of vegetable marrow, or, as already men- 

 tioned, 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 pro- 

 duce it, it would be obviously just as certain in this 

 case that the floating dust contains the germs of Peni- 

 cillium 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 



