154 APPLICATION OF PRINCIPLEg. 



ration 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 instruments give no indication. To 

 counteract these mischievous effects, many contri- 

 vances have been proposed, in order to ensure the 

 introduction of fresh air warm and loaded with mois- 

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

 after passing through pipes moderately heated, or 

 over hot-water pipes surrounded by a damp atmo- 

 sphere, 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 purifying the air, as is often the case in hot-houses 

 and in dung-pits, it should be effected by the intro- 

 duction of fresh air, damp and heated. 



If it is only for the purpose of lowering the tempera- 

 ture, as in green-houses, 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 Holland plants : in these cases 

 it should be brought into the house as near the tem- 

 perature 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 



