LABORATORY WORK 183 



the bottle, ammonia may be detected by its smell and the other 

 tests described above. If not, add dry sodium carbonate, draw a 

 current of air through the warmed mixture, and then through water 

 made acid (blue) to congo red. This colour will turn red quickly, 

 and more acid may be added from time to time. Fig. 12 shows the 

 arrangement. 



Carbon Cycle 



Water Culture. The solution to be used may consist of the 

 following salts : 



Calcium nitrate - 4 gm. 



Potassium nitrate - i 



Magnesium sulphate - i 



Acid potassium phosphate I 



Potassium chloride 0.5 



Tap water 3 litres 

 A drop or two of dilute ferrous sulphate solution 



A large glass jar is provided with a wooden lid having a hole in the 

 middle. The seedling of a Windsor bean, selected from a number 

 which have been allowed to germinate between wet filter paper, is 

 gently supported in the hole by means of bits of cork, so that the 

 root dips into the solution. The wooden cover and the corks 

 should have been soaked in melted paraffin wax. The weight of 

 the bean, dried in air, is noted before germination. After the plant 

 has grown to a foot or more in height, it is removed, allowed to dry 

 in the air, and weighed again. 



For success, growth must take place in a good light, the root 

 being kept dark by a covering of brown paper on the jar. If 

 exposed to the sun, the jar should be in a box filled with sawdust 

 to prevent the solution becoming hot. The solution is changed at 

 a few days' interval, and air blown through it occasionally to supply 

 oxygen to the roots. 



The experiment, of course, requires some weeks for completion. 



Action of the Green Plant on Carbon Dioxide 



Make first an analysis of atmospheric air. This may be 

 done with the Hempel burette and two Hempel pipettes, one 

 for caustic soda, the other for alkaline pyrogallol. For our pur- 

 pose, it will be simpler to use a nitrometer tube, connected with 

 a reservoir of mercury, and to perform the analysis in the tube 

 itself. The arrangement is represented in Fig. 13. The tube A 

 is graduated into o. i c.c. and can be connected either with the 

 funnel 15 or the tube C, or closed altogether, by means of the 

 3- way stopcock. A thick-walled rubber tube is attached to the 



