50 



THE FLOATING-MATTER OF THE AIR. 



middle by a hole 2 inches in diameter, closed air-tight 

 by a sheet of india-rubber. This sheet is pierced in the 

 middle by a pin, and through the pin-hole is passed the 

 shank of a long pipette ending above in a small funnel. 



A circular tin collar, 2 inches in diameter and 1| inch 

 deep, surrounds the pipette, the space between both 

 being packed with cotton-wool moistened by glycerine. 

 Thus the pipette, in moving up and down, is not only 

 firmly clasped by the india-rubber, but it also passes 



