18 EFFECT OF CO 2 . [CH. I 



provides for the overflow. The object is mounted between 

 two cover-glasses, which are gently clamped together by a 



FIG. 6. Exp. 17. 



bit of tinfoil of the form shown in fig. 6, the flaps being 

 bent up at 45 along the dotted lines. Unless some such 

 plan is adopted, the upper cover-glass is liable to be 

 washed off by currents in the water. 



The same method may be used to subject circulating 

 protoplasm to a low temperature. 



(18) Effect of C0 2 . 



To observe the effect of gases on circulating proto- 

 plasm, the Elodea leaf is mounted in a small drop of 

 water, on the under surface of a cover-glass forming the 

 roof of a gas chamber: if the cover-glass projects fairly 

 well beyond the edges of the hole on which it lies, the 

 apparatus can be made sufficiently gas-tight by painting 

 the edges of the cover-glass with olive oil ; or the glass 

 may be fixed with putty. Having under observation a 

 circulating cell, attach the tube of the gas chamber to the 

 C0 2 -generating apparatus 1 , and observe that the proto- 



1 The C0 2 must be made to bubble through water before it reaches 

 the gas chamber. 



