PHOSPHATES AND NITROGEN 89 



"To finish this difficult but very important subject 

 of the nutrition of plants, I must say a few words 

 about ammonia. This word does not tell you any- 

 thing since it is a new word to you. But I will make 

 its meaning clear to you by a familiar illustration. 



"You must have noticed the strong, penetrating 

 odor prevalent in ill-kept water-closets; and you 

 have also perceived the same odor when soiled gar- 

 ments are cleaned with a certain liquid that looks 

 like clear water. Well, this odor, so pungent that it 

 almost produces the effect of fine needles thrust up 

 into the nostrils and brings tears to the eyes, is the 

 odor of ammonia. 



"Ammonia is an invisible gas capable of being 

 taken up in large quantities by water, the mixture 

 being known as aqua ammonia, or water of ammonia. 

 Combined with other substances ammonia loses its 

 pungent odor and forms compounds which are among 

 the most effective fertilizers. These compounds fur- 

 nish vegetation with one of its essential ingredients 

 called nitrogen. By itself nitrogen is an odorless 

 and colorless gas. In this state it forms four-fifths 

 of the volume of ordinary air, the air we breathe; 

 The other fifth is composed of a second gas called 

 oxygen, also colorless and odorless. It is oxygen 

 that our lungs demand when we breathe, and it is 

 oxygen that is necessary when we wish to burn any- 

 thing. It is this alone that plays its invaluable part 

 in the combustion of certain substances in our blood 

 and in the generation of natural heat ; it is this that 



