ON VEGETABLES. 123 



slant it is formed. Such a state of stimulus might be fol- 

 lowed by a debilitating re-action, as is the case with us, 

 after excessive exercise ; but, while it lasts, it must pro- 

 duce a very great increase of action throughout the frame. 



Mrs. B. That may possibly be a concurring cause of 

 the phenomenon, but it is not sufficient to "account wholly 

 for it. Mr. De Candolle mentions the remarkable growth 

 of the branch of a vine during a storm, of no less than an 

 inch and a quarter in the course of an hour and a half; 

 now the tree grew against a wall, so as to be little acces- 

 sible to the wind. 



The quantity of water contained in the atmosphere is 

 a point of great importance to plants. Water, you may 

 recollect, exists in the atmosphere in two different states : 

 in the one it is so completely dissolved, that the air feels 

 perfectly dry to us, and affords no moisture to the vegeta- 

 ble part of the creation. It is heat which enables the air 

 to perform this solution ; therefore the higher its temper- 

 ature the more water it can dissolve. 



Caroline. Then the atmosphere in the torrid zone, 

 though driest, contains most water : that appears very 

 paradoxical. 



Mrs. B. It is nevertheless true. Whenever the air 

 cools, its power of retaining water in solution diminishes. 



Caroline. This must happen, then, not only when the 

 weather changes from hot to cold, but every evening after 

 sunset. 



Mrs. B. Accordingly, we continually see misty vapors 

 floating in the atmosphere in the evening, and the ground 

 more or less covered with dew : all this is water precip- 

 itated by the diminution of temperature of the air. In 

 the morning, when the sun has sufficiently warmed the 

 atmosphere to enable it to dissolve these fogs and vapors, 

 they disappear. 



The other state in which water exists in the atmosphere 

 is that of a tine subtle vapor, diminishing its clearness, 

 and giving us the sensation of humidity. 



Emily. The latter state, I should suppose, would be 



686. What does Candolle say of tliis subject! 687. And of the 

 water contained in the atmosphere 1 688. In what region does the 

 atmosphere contain most water! 689. How is the dew at evening pro- 

 duced! 690. What results to the dew from the warming of the at- 

 mosphere in the morning 1 ? 691. In what other state does water ex- 

 ist in the atmosphere! 



