CHAPTER V. 



WATER CULTURE, PHOTOSYNTHESIS, RE- 

 SPIRATION. 



I. ASH ANALYSIS AND WATER CULTURE. 



214. Proportion of Water in Fresh Tissues. In 



order to analyse a plant, it is necessary to determine the 

 percentage weights of (1) the water it contains in the fresh 

 state, (2) the carbon present in the dried material, (3) the 

 incombustible ash left after strongly heating the dried 

 material. In order to determine the proportion of water, 

 and at the same time the " dry weight " of the plant, it is 

 only necessary to dry the fresh plant thoroughly without 

 charring it. If the plant is now burnt, the carbon, hydro- 

 gen, and nitrogen which it contains are given off in the 

 form of gases (carbon dioxide, water, oxides of nitrogen) ; 

 hence these three elements may not be present in the ash 

 that remains after complete combustion. 



215. Carbon and Ash. Dry and weigh a porcelain 

 crucible or evaporating dish, place in it half of the oven- 

 dried leaves, and weigh the crucible again, to obtain the 

 weight of the dried leaves placed in it. Heat the crucible 

 strongly : the dry material chars, and in ten minutes or so 

 it is reduced to fine ash, which should not be allowed to 

 glow. Find the weight of the ash, weighing the crucible 

 and its contents twice or thrice until no further loss occurs 

 on heating it. To ascertain roughly the amount of carbon, 



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