14 CARBON DIOXIDE. [CH. I 



however, should not be used with a valuable microscope. 

 The objective and the preparation are immersed in water 

 contained in a glass dish standing on the stage of the 

 microscope. A siphon, provided with a tap, allows warm 

 water to run into the dish, while a second siphon and tap 

 provides for the overflow. The object is mounted between 

 two cover-slij)s, which are gently clamped together by a 

 bit of tinfoil of the form shown in fig. 8, the flaps being 

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

 plan is adopted, the upper cover-slip is liable to be washed 

 off by currents in the water. 



(17a) The same method may be used to subject cir- 

 culating protoplasm to a low temperature. 



(18) Effect of CO,. 



To observe the effect of gases on circulating protoplasm, 

 the Elodea leaf is mounted in a small drop of water, on 

 the under surface of a cover-slip forming the roof of a gas 

 chamber: if the cover-slip projects fairly well beyond the 

 edges of the hole on which it lies, the apparatus can be 

 made sufiiciently gas-tight by painting the edges of the 

 cover-slip with olive oil; or the slip may be fixed with 

 putty. Having under observation a circulating cell, 

 attach the tube of the gas chamber to the COj-generating 

 apparatus^ and observe that the protoplasm comes to rest: 

 by disconnecting and allowing air to pass, the circulation 

 can be renewed. 



1 The CO2 must be made to bubble through water before it reaches 

 the gas chamber. 



