ATMOSPHERIC AIK AS THE FOOD OF PLANTS. 49 



plants ill wliich the microscope reveals chlorophyll, altliough 

 the prevailing color may be other than green. 



Influence of Lij?ht on Fixation of f arbon. — As men- 

 tioned, Iiigenhouss (in 1779) discovere'l that oxygen gas 

 is given olF from foliage, and carbon fixed in the i)lant 

 only under the influence of light. Experiments show that 

 when a seed germinates in exclusion of light it not only 

 does not gain, but steadily loses weight from the consump- 

 tion of carbon (and hydrogen) in slow oxidation (respira- 

 tion). 



Thus Boussingault {Comptes Rendus, 1864, p. 883) 

 caused two beans to germinate and vegetate, one in the 

 ordinary light and one in darkness, during 26 days. The 

 gain in light and loss in darkness in entire (dry) weight, 

 and of carbon, etc., are seen from the statement below. 



In L'ujht. Ill Darkness. 



Weight of seed 0.933 gnim ... .0.936 gram. 



Weight of plant 1.293 " 0.566 " 



Gain = 0.371 gram. Loss 0.360 gram. 



Carbon, Gain = 1926 " Loss. . .0.1598 " 



Hydrogen, " = 0.0200 " " ...0.0232 " 



Oxygen, " = 0.1591 " " ...0.1766 " 



§6. 



THE AMMONL\ OF THE ATMOSPHERE AND ITS RELATIONS 

 TO VEGETABLE NUTRITION. 



Ammonia is a gas, colorless and invisible, but having a 

 peculiar pungency of odor and an acrid taste. 



PiH'parsitioii. — It may be obtained in a state of purity by licat- 

 ing a mixture of chloride of ammonium (sal ammoniac) and (luielvlime. 

 Equal qu;intities of tlie two substances just named (50 grams of each) 

 are separately pulverized, introduced into a flask, and well mixed by 

 shaking. A straight tube 8 inches long is now secured in the neck of 

 the flask by means of a perforated cork, and heat applied. The ammonia 

 gas which soon escapes in abundance is collected in dry bottles, which 

 are inverted over the tube. The gas, rapidly entering the bottle, in a 

 few moments displaces the twice heavier atmospheric air. As soou as » 



3 



