THE SCOPE OF BIOLOGY 



15 



Recognizing that the great source of error in these experi- 

 ments was due to the germ-bearing dust of the air, Tyndall 

 attempted to free the 

 air from dust by coating 

 the inside of the curved 

 tubes with glycerine, to 

 entangle the dust particles 

 of the air as it passed up 

 and down the series of 

 curves into the box. This 

 method proved to be suc- 

 cessful, for experiment and 

 microscopic study showed 

 that no dust passed be- 

 yond the second curve of 

 the tubes. The interior of 

 the box was also coated 

 with glycerine, so that the 

 dust particles which either 

 settled to the bottom or 



FIG. 3. APPARATUS USED BY TYNDALL 



For description see text. 



floated against the side or top of the box would be caught in the 

 glycerine. In this way Tyndall argued that he could obtain, 

 in time, air perfectly free from germ-bearing particles. 



He did not wish to begin an experiment until the air in the 

 box was absolutely free from dust, and in order to determine 

 this point the two glass windows at the end of the box were 

 used. A ray of light was thrown through the box, in at one 

 window, and out through the other. Thus, any dust particles 

 that remained floating in the air of the box would be illumined 

 and made clearly visible through the window in front. At 

 first there were many dust particles to be seen floating in the 

 air; but after the box had remained quiet for several days, 

 the ray of light was invisible as it passed through the box, 

 proving that no floating dust particles were present to be 

 illumined. When this condition was reached Tyndall assumed 



