FERMENTATION. 267 



thistle-seeds form, at all events, part of the powder/ 

 Supposing a succession of such powders to be placed in 

 your hands with grains becoming gradually smaller, 

 until they dwindle to the size of impalpable dust parti- 

 cles; assuming that you treat them all in the same way, 

 and that from every one of them in a few days you ob- 

 tain a definite crop it may be clover, it may be mus- 

 tard, it may be mignonette, 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 conclusion. Without a shadow of 

 misgiving you would conclude that the powder must 

 have contained the seeds or germs of the life observed. 

 There is not in the range of physical science, an ex- 

 periment 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 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 

 enough 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, want- 

 ing 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 

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