THE MOLECULAR FORCES IN PLANTS. 163 



volume of the gases is effected during the experiment by con- 

 ditions of temperature and atmospheric pressure. It is now seen 

 after some time that the mercury in the tube closed with the 

 membrane stands considerably higher than the other, and this is 

 due to the fact that owing to the absorption of the Carbon dioxide 

 by the rubber, the gas, in spite of its high specific gravity, passes 

 more rapidly through the membrane than can the atmospheric 

 air. 



I closed a glass tube at one end with a fresh piece of Nerium 

 Oleander leaf, filled the tube with Carbon dioxide, and inverted it 

 under mercury. The mercury rose fairly high in the tube. In 

 this experiment the best way to make the tube air-tight is to slip 

 over it a piece of cork with a hole in the middle, till the top of 

 the tube comes exactly flush with the surface of the cork, and fix 

 this in position, so as to serve as a rim, by means of sealing-wax 

 applied below. The cork rim is then smeared on the top with a 

 mixture of 1 part of yellow wax, 1 part of olive oil, and 1 part 

 of melted mutton suet. The piece of leaf is placed on it with its 

 upper surface, which is free from stomata, downwards, and the 

 operation is completed by smearing the edges of the leaf with the 

 cement. 



More accurate results are obtained by the following method of 

 investigation. A perforated piece of cork or elder pith is pushed 

 over the well-ground end of a glass tube 5-6 mm. wide, and 50- 

 100 cm. long, so as to form a rim. We warm well and smear it 

 with melted sealing- wax of the finest quality. -We now lay the 

 object to be investigated {e.g. a dry, perfect piece of Hedera Helix 

 leaf, the upper side of which, as is known, is free from stomata, 

 or a thin lamella of cork, cut, it may be, with the microtome) on 

 a piece of cork, invert the glass tube, and bring it with moderate 

 pressure in contact with the tissue while the sealing-wax is still 

 melted. As a rule, we get in this way a perfectly air-tight 

 closure. After cooling thoroughly, the tube is laid nearly hori- 

 zontal, carefully filled entirely or only in part with mercury, and 

 then fixed vertically with its open end dipping under mercury. 

 We now displace part of the mercury in the tube by dry Carbon 

 dioxide, and let the apparatus stand in a place looking to the 

 north. The position of the mercury in the tube is read, with the 

 necessary precautions, the temperature and barometric pressure 

 being simultaneously noted ; and if we repeat this frequently, e.g. 

 once every day, we obtain the following" result. In the apparatus 



