PASTEUR, KOCH, AND OTHERS 291 



bcr would close the hole again, but it would also admit of 

 the passage through it of a small glass tube, such as is 

 called by chemists a " thistle tube." The interior of this 

 box was painted with a sticky substance like glycerin, 

 in order to retain the floating particles of the air when they 

 had once settled upon its sides and bottom. The apparatus 

 having been prepared in this way, was allowed to stand, and 

 the floating particles settled by their own weight upon 

 the bottom and sides of the box, so that day by day the 

 number of floating particles became reduced, and finally all 

 of them came to rest. 



The air now differed from the outside air in having been 

 purified of all of its floating particles. In order to test the 

 complete disappearance of all particles. Tyndall threw a 

 beam of light into the air chamber. He kept his eye in the 

 darkness for some time in order to increase its sensitiveness; 

 then, looking from the front through the glass into the box, 

 he was able to see any particles that might be floating there. 

 The floating particles would be brightly illuminated by the 

 condensed light that he directed into the chamber, and 

 would become visible. When there was complete darkness 

 within the chamber, the course of the beam of light was 

 apparent in the room as it came up to the box and as 

 it left the box, being seen on account of the reflection from 

 the floating particles in the air, but it could not be seen 

 at all within the box. When this condition was reached, 

 Tyndall had what he called optically pure air, and he was 

 now ready to introduce the nutrient fluids into his test tubes. 

 Through a thistle tube, thrust into the rubber diaphragm 

 above, he was able to bring the mouth of the tube successively 

 over the different test tubes, and, by pouring different kinds 

 of fluids from above, he was able to introduce these into 

 different test tubes. These fluids consisted of mutton broth, 

 cf turnip-broth, and other decoctions of animal and vcgeta- 



