284 FRAGMENTS OF SCIENCE 



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 mechan- 

 ically 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 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 con- 

 taining precisely the same liquid to the dust-laden air. 

 In three days every one of the latter stinks, and, exam- 

 ined with the microscope, every one of them is found 

 swarming with the bacteria of putrefaction. After three 

 months, or three years, the beef -tea within the chamber 

 is found in every case as sweet and clear, and as free 

 from bacteria, as it was at the moment when it was first 

 put in. There is absolutely no difference between the air 

 within and that without save that the one is dustless and 

 the other dust-laden. Clinch the experiment thus: Open 

 the door of your chamber and allow the dust to enter it. 

 In three days afterward you have every vessel within the 

 chamber swarming with bacteria, and in a state of active 

 putrefaction. Here, also, the inference is quite as certain 

 as in the case of the powder sown in your garden. Mul- 

 tiply your proofs by building fifty chambers instead of 



