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, oi 
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, 
