140 THE POPULAR SCIENCE MONTHLY. 



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 impal- 

 pable 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 migno- 

 nette, 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 experiment 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 Penicillmm glaucum, to which I have already re- 

 ferred, are light enough to float in the air. A cut apple, a pear, a to- 

 mato, a slice of vegetable marrow, or, as already mentioned, an old 

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

 for the Pe?iicillium. 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 win- 

 dow-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 abso- 

 lutely, 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 illu- 

 minated and observed, is as palpable to sense as any 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 or ceiling, until, finally, by 

 this self-cleansing process, the air is entirely freed from mechanically 

 suspended 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 



