186 INTRODUCTION TO GENERAL PHYSIOLOGY 



dioxide decreased, or nearly disappeared. Thus the green plant 

 in sunlight restores the normal composition to air which has been 

 vitiated by respiratory processes. 



The experiment may be repeated in the dark. There will be 

 no increase in oxygen. Probably a decrease will be noted, together 

 with an increase in carbon dioxide. 



Chlorophyll. To see the absorption bands in the spectrum of 

 chlorophyll, take grass cut in short pieces, and rub in a mortar with 

 methylated spirit. Filter and place in a test-tube in front of the 

 slit of a pocket spectroscope. If the dark band in the red is not 

 clearly seen, dilute with spirit. 



The chloroplasts are most easily seen under the microscope in a 

 thin leaf, such as that of a moss. Mount in water. 



Formation of Starch. Take a plant, a bean plant does well, 

 growing in a pot. Keep it in the dark for two days. The starch 

 will be transported from the leaves to the stem and root. Take one 

 of the leaves, dissolve out the chlorophyll by warming in methylated 

 spirit, and place the colourless leaf, after washing with water, in a 

 dilute solution of iodine in potassium iodide. It will not turn blue. 

 Allow the plant to be exposed to sunlight for a day or two, and test 

 the leaf for starch again. It will turn blue. The iodine test should 

 also be made with a solution of starch, made by rubbing some dry 

 starch powder in water and pouring into boiling water. 



The Nitrogen Cycle 



Bacteria Allow hay or grass to putrefy in water. Various 

 forms of bacteria will be found on examination under the micro- 

 scope. Use the highest magnifying power available. 



Their forms and movements can be made more obvious by the 

 addition of indian ink, or better, collargol, to the preparation, or 

 by the use of dark-ground illumination. 



Formation of Nitrates in the Soil. Add half a gram of garden 

 soil to 50 c.c. of the following culture fluid : 



Ammonium sulphate - 0.5 gm. 



Potassium acid phosphate I 



Water - i litre. 



Half a gramme of magnesium carbonate to the 50 c.c. is also 

 required to preserve neutrality. After about four weeks or so, the 

 ammonia will have disappeared, and nitrate have taken its place. 

 The presence of nitrate may be shown as follows : Filter the 

 liquid. Evaporate to dry ness in a porcelain basin on the water- 

 bath. Add, with a glass rod, a drop of 0.5 per cent, solution of 

 diphenylamine in pure, nitrate free, strong sulphuric acid. The 

 presence of nitrate is shown by the production of aniline blue. 



