WATER CULTURE, PHOTOSYNTHESIS, RESPIRATION. 177 



decolorised methylene blue to resume its blue colour, 

 (4) the production of white fumes of phosphorus oxide 

 from phosphorus. 



For volumetric experiments, carbon dioxide is best 

 absorbed by caustic potash, and oxygen by pyrogallate 

 of potash. A 1 in 3 solution of caustic potash (1 gram 

 KHO to every 3 c.c. water) will absorb at least thirty 

 times its volume of carbon dioxide in a few minutes if 

 well shaken up with the gaseous mixture to be analysed ; 

 the solid sticks weigh roughly 1 gram to the centimetre, 

 so it is only necessary to measure off the length of stick 

 required in making up the 1 in 3 solution. For absorp- 

 tion of oxygen, dissolve 1 gram of pyrogallic acid and 

 5 grams potash in every 30 c.c. of water ; the solution 

 should be made up just before use, by mixing equal 

 volumes of (a] 1 in 15 solution of 

 pyrogallic acid in water and (fe) 5 

 in 15 solution of caustic potash in 

 water ; the pyrogallate of potash 

 solution thus made will absorb about 

 ten times its own volume of oxygen. 



234. Ganong's Pliotosyiitliometer. 

 For the quantitative study of the two 

 gases concerned in photosynthesis it is 

 necessary to keep a leafy shoot, or single 

 leaves, or an entire plant, in a closed 

 chamber, expose the apparatus to light, 

 and analyse the enclosed gases to deter- 

 mine the increase of the oxygen (20 per 

 cent.) and the diminution of the carbon di- 

 oxide (0*004 per cent.) that were present in 

 the air at the beginning of the experiment. 



Since the percentage of carbon dioxide 

 in the atmosphere is so small, arid plants 

 will thrive for a time in air containing a 

 much larger percentage, up to 10 per cent, 

 or even more, we can add a selected per- 

 centage of this gas to the air in the vessel 

 by means of a generator (or a Sparklet 

 siphon charged without addition of water). 



Prof. Ganong's Photosynthometer, large enough for a shoot of a 

 small-leaved plant, is shown in Fig. 41. It is supplied by the 

 Bausch and Lomb Optical Company, with full instructions for use. 

 P, B. 12 



Fig. 41. Ganong's Photo- 

 synthometer. 



