THE FOOD OF PLANTS. 43 



mercury must rise in the apparatus. The connection tubing is 

 closed at the proper time by means of a clip. We then intro- 

 duce into it a piece of glass rod, ground and greased at its lower 

 end, remove the clip, and push the glass rod home, so that its 

 greased end fits closely to the top of the tube a. 



The mercury having been sucked up into the tube as described, 

 and the temperature of the gas having after some time become 

 uniform, we can proceed to the necessary readings. We read off 

 the volume of the gas, counting to the bottom of the water 

 meniscus, and note also the height of the layer of water, and the 

 height of the mercury above the surface. The height of the 

 column of mercury expressed in millimetres, together with the 

 mercurial pressure corresponding with the layer of water, gives 

 the deduction to be made from the barometric reading. Tem- 

 perature and barometric pressure must of course be accurately 

 noted. From the gas volume observed is to be deducted the 

 volume of the wire and leaf, which we determine in the usual 

 way by immersion in water, and also 0'3 c.c. for the water 

 meniscus. The volume is now reduced to C., 1000 mm. pres- 

 sure of mercury, and the condition of dryness : 3 



(l + 0'00366t) 



V is the reduced gas volume ; V denotes the volume observed ; 

 m, the meniscus correction ; 6, the barometric pressure ; 6', the 

 pressure height to be deducted for the column of mercury in 

 the tube ; 6", the tension of aqueous vapour at a temperature 

 of* . 



Wo now lead into the apparatus some purified Carbon dioxide 

 (say 8 c.c.), once more determine the volume of gas in the 

 apparatus, and again reduce as above. We can then at once find 

 the volume of Carbon dioxide introduced by subtracting the first 

 corrected reading from the second. In introducing the Carbon 

 dioxide, and also in the previous operations, care must be taken 

 to touch the apparatus as little as possible, so that the equalisation 

 of temperature necessary before taking the readings may be 

 effected as quickly as possible (in, say, ten to twenty minutes). 



now carefully wash with water. If necessary, these operations are to be re- 

 peated. We then dry the mercury with blotting paper, heat it in a dish to 120 C. 

 in a draught chamber, cover the dish with a sheet of paper, and allow to cool. 

 Fimilly the metal is filtered through writing paper pierced here and there by 

 means of a needle. 



