42 PLANT PHYSIOLOGY. 



and to place on it a cm. of sulphuric acid and mercurous sulphate. After 

 use the mercury may be poured in at the top, and may be drawn off dry 

 and pure below. 



To Remove Carbon Dioxide from a Closed Space. Hold the recep- 

 tacle containing the gas over a small dish of water, and add to the latter 

 some caustic potash, when the liquid will rise, absorbing the gas. This 

 method also affords a fair test of the quantity of the gas contained in a 

 given space. 



To Test for tJie Presence of Carbon Dioxide. If in considerable quan- 

 tity, the method described for its absorption may be used. If in smaller 

 quantity, some filtered lime, or baryta, water brought into contact with it 

 will turn milky if the gas is present. In air containing about three per 

 cent or more of the gas a candle will not burn. 



To Generate Carbon Dioxide. Prepare a large wide-mouthed bottle 

 as shown in Fig. 7, with a tight cork through which passes a thistle-tube 

 and a plain tube. Place pieces of marble, limestone, or chalk in it and 

 cover them with water, below the surface of which the thistle-tube is to 

 dip. From the plain tube, which ends just below the cork, lead a rub- 

 ber tube to the place where the gas is needed, and pour a little hydro- 

 chloric acid through the thistle-tube, when the carbon dioxide will come 

 off copiously, though at first much mixed with air. For most purposes it 

 is well to add a wash-bottle to prevent acid going over with the gas. For 

 this use a small bottle with a tight cork through which pass two glass 

 tubes, one dipping below the surface of an inch of water. Connect the 

 plain tube of generator and the long tube of the bottle by a rubber tube, 

 and lead a rubber tube from the short tube of the bottle to the place of 

 use of the gas. In using the apparatus, time must be allowed for the gas 

 to drive out the air. For some investigation purposes this arrangement 

 does not give a sufficiently pure gas, which may be obtained as described 

 in all works on chemistry. 



To Test for the Presence of Oxygen. If the quantity is proportion- 

 ally much larger than occurs in the atmosphere, a glowing splinter thrust 

 into it will burst into flame. Or phosphorus (thin sticks especially useful 

 for the purpose are obtainable) will give off white fumes in a space con- 

 taining oxygen ; if over water, these fumes are absorbed by it, allowing 

 it to rise into the vessel. Or, the pyrogallic method described in the 

 next section may be used. The phosphorus must be kept always under 

 water when not in use, and must not be touched by the hands. In test- 

 ing it may best be used in a cage of wire netting on the end of a copper 

 wire (Fig. 7). The use of several thin sticks at once will give the 

 oxygen test very quickly. 



To Remove Oxygen from a Closed Space. Plunge the open mouth of 

 the vessel containing it just under the surface of water in a small dish, 

 and add to the water enough well-mixed caustic potash and pyrogallic 



