PASTEUR, KOCH, AND OTHERS 291 
ber 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, 
of turnip-broth, and other decoctions of animal and vegeta- 
