SPONTANEOUS GENERATION. 
579 
We have now to devise a means of testing the action of 
such spontaneously purified air upon putrescible infusions. 
Wooden chambers, or cases, are accordingly constructed, 
having glass fronts, side-windows, and back-doors. 
Through the bottoms of the chambers test-tubes pass air- 
tight; their open ends, for about one-fifth of the length of 
the tubes, being within the chambers. Provision is made 
for a free connection through sinuous channels between 
the inner and the outer air. Through such channels, 
though open, no dust will reach the chamber. The top of 
each chamber is perforated by a circular hole two inches in 
diameter, closed air-tight by a sheet of india rubber. This 
