DOMESTIC GREENHOUSES. 



95 



the soil be pretty well saturated, and the liquid begins to run off 

 by the two openings in the bottom. After draining thus for 

 twenty-four hours, cork up the holes, place the glass-case on the 

 box, and the operation will be finished. 



After the first preparation, the plants require little or no care ; 

 the case need only be opened for the removal of dead leaves, or 

 for a little trimming, when required. Plants in open flower-pots 

 are exposed to the vicissitudes of change of climate, and require 

 constant watering ; but the plants in these cases seem to be in- 

 dependent of any change of temperature in the air, and water 

 themselves. The moisture rises by the sun's influence from the 

 moistened earth, cherishes the leaves of the plants in its aerial 

 condition, and during the cool of night falls to the earth again 

 like rain or dew. In this manner there is a constant succession 

 of rising and falling of moisture, in imitation of the great processes 

 of nature, daily going on in the fields around us. The plant-case 

 is a little world in itself, in which vegetation is supported solely 

 by the resources originally communicated to it. 



Not the least remarkable part in the economy of the case is 

 the preservation of atmospheric purity. To all who reflect for 

 the first time on this subject, it will seem incomprehensible how 

 the plants can possibly thrive and blossom without the occasional 

 interchange of fresh air with the atmosphere. This certainly does 

 appear extraordinary, yet it is ascertained by experiment that no 

 such reinvigoration is requisite. To account for the phenomena, 

 it will be necessary to explain the constitution of atmospheric air, 

 and the means adopted by nature for its purification. 



Air consists of three gases in close mechanical union — nitrogen, 

 oxygen and carbonic acid, in the proportion of about 79 of nitro- 

 gen, 20 oxygen, and 1 of carbonic acid, in 100 parts of pure air. 

 In this mixed composition, the essential element for the support 

 of respiration in both animals and plants, and also for combustion, 



