268 FKAG-MENTS OF SCIENCE. 



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 per- 

 fectly 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, 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 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 just covered with warm 

 water ; you thus extract the juice of the beef in a con- 

 centrated form. By properly boiling the liquid and 

 filtering it, you can obtain from it a perfectly trans- 

 parent beef- tea. Expose a number of vessels containing 

 this tea to the moteless air of your chamber ; and 

 expose a number of vessels containing precisely the same 

 liquid to the dust-laden air. In three days every one of 

 the latter stinks, and examined 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 abso- 

 lutely no difference between the air within and that with- 

 out save that the one is dustless and the other dust- laden 



