OF VENTILATION. 1613 



quantity of whicli is enough to deteriorate the air, so 

 far as vegetation is concerned. Drs. Turner and 

 Christison found that Toroo of sulphurous acid gas 

 destroyed leaves in forty-eight hours ; and similar 

 effects were obtained from hydro-chloric or muriatic 

 acid gas, chlorine, ammonia, and other agents, the 

 presence of which was perfectly undiscoverable by 

 the smell. We also know that the destructive pro- 

 perties of air poisoned by corrosive sublimate, per- 

 haps by its being dissolved in the vapour of a hot- 

 house, are not at all appreciable by the senses. 



Yentilation is necessary, then, not to enable plants 

 to exercise their respiratory functions, provided the 

 atmosphere is unmixed with accidental impurities ; 

 but to carry off noxious vapours generated in the arti- 

 ficial atmosphere of a glazed house, and to produce 

 dryness, or cold, or both. 



When the external air is admitted into a glazed 

 house containing a moist atmosphere, it, under ordi- 

 nary circumstances, is much colder than that with 

 which it mixes ; the heated damp air rushes out at 

 the upper ventilators, and the drier cold air takes its 

 place; the latter rapidly abstracts from the plants 

 and the earth, or the vessels in which they grow, a 

 part of their moisture, and thus gives a sudden shock 

 to their constitution, which cannot fail to be injurious. 

 This abstraction of moisture is in proportion to the 

 rapidity of the motion of the air. But it is not mere- 

 ly 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 evapo- 



7* 



