WINTER VENTILATION. 223 



constitution, which cannot fail to be injurious. This abstrac- 

 tion of moisture is in proportion to the rapidity of the motion 

 of the air. But it is not merely dryness that is thus produced, 

 or such a lowering of temperature as the thermometer 

 suspended in the interior of the house may indicate ; the rapid 

 evaporation that takes place upon the admission of dry air 

 produces a degree of cold upon the surface of leaves, and of the 

 porous earthen pots in which plants grow, of which our instru- 

 ments give no indication. To counteract these mischievous 

 effects many contrivances have been proposed, in order to 

 insure the introduction of fresh air warm and loaded with 

 moisture, such as compelling the fresh air to enter a house 

 after passing through pipes moderately heated, or over hot- 

 water pipes surroimded by a damp atmosphere, and so on, the 

 advantages of which, of course, depend upon the objects to be 

 attained. 



If ventilation is merely employed for the purpose of puri- 

 fying the air, as is often the case in hothouses and in dung 

 pits, it should be effected by the introduction of fresh air damp 

 and heated. If it is only for the purpose of lowering the 

 temperature, as in greenhouses, or in the midst of summer, 

 the external air may be admitted without any precautions. 



But it is very commonly required in the winter, for the 

 purpose of drying the air in houses kept at that season at a 

 low temperature; such, for instance, as those built for the 

 protection of Heaths, and many other Cape and New HoUand 

 plants : in these cases it should be brought into the house as 

 near the temperature of the house as possible, but on no 

 account loaded with moisture. One of the principal reasons 

 for drying the air of such houses is, to prevent the growth of 

 parasitical fungi, which, in the form of mouldiness, constitute 

 what gardeners technically call " damp." These productions 

 flourish in damp air at a low temperature, but wiU not exist 

 either ia dry cold air or in hot damp air. If the air of cool 

 greenhouses is allowed to become damp, the fungi immediately 

 spring up on the surface of any decayed leaves, or other matter 

 which may be present, when tibey spread rapidly to the young 

 and tender parts of hving plants ; and when this happens they 



