X CONTENTS. 



III. Influence of the ammonia of the atmosphere . . .80 

 Sources of ammonia. 



1. Putrefaction of animal bodies . . , . , .81 



2. Decay of vegetable substances ...... 81 



3. Volcanoes 81 



IV. Nitric acid of the atmosphere 82 



V. Light carbureted hydrogen 83 



VI. Influence of the carbonic acid of the atmosphere . . 83 

 Source of carbovic acid. 



1. Chemical action 84 



2. Combustion 84 



3. Respiration of animals ■ . 84 



4. Decay of vegetables 84 



Quantity of carbonic acid in the atmosphere . . . .84 

 "W%at becomes of this acid ? 85 



1. It is decomposed by vegetation . . . . .85 



2. Quantity of this acid absorbed at different periods . . 8G 

 Other causes which abstract it . . . , . . .88 

 Necessity of carbonic acid to vegetation . . . . .88 



VII. Mechanical agency of the atmosphere. 



1. Pressure 87 



2. Medium for the action of other agents . . . .90 

 Beautiful and wise constitution of the atmosphere . . .91 



Sect. 2. Agency of water upon the vital functions of plants. 



I. Water in the solid form — ice and snow 02 



1. Benefits of freezing the soil 92 



2. Protection afforded the roots by ice and snow . . .92 



II. "Water in the liquid form. 



1. Its solvent properties ....... 93 



2. Its chemical agency 94 



3. Its mechanical agency 95 



4. Its agency as nutriment 95 



III. Water in the state of vapor. 



1. Quantity of vapor in the atmosphere . . . .96 



2. Absorption by the leaves and roots , . . . .96 



3. Influence in dry seasons ....... 97 



4. Agency of dew as affected by the conducting power . 97 



5. Agency of rain ......... 97 



6. Effect of evaporation upon soil ... .98 



