EFFECTS OF VENTILATION. 265 



While, however, the natural atmosphere of a hot-house can- 

 not be supposed to require changing, in order to adapt it to the 

 respiration of plants, it is to be borne in mind that the air of 

 hot-houses, artificially heated, may be rendered impure by the 

 means employed to produce heat, as will be seen from what has 

 already been said on the principles of heating, in the preceding 

 part of this work. Sulphuric acid gas, in variable quantities, 

 escapes from brick flues, especially old and imperfectly con- 

 structed ones, and various other unsuspected sources of impu- 

 rity, an infinitely small quantity of which is sufficient to con- 

 taminate the air, in respect to vegetable life. 



Drs. Turner and Christison found that TT njui7 of sulphurous 

 acid gas destroyed leaves in forty -eight hours ; and similar effects 

 were observed from hydro-chloric acid gas. Chlorine, ammo- 

 nia, and other gases produce the same results, when their pres- 

 ence is altogether undiscoverable by the olfactory organs. We 

 also know that the destructive properties of air, poisoned by cor- 

 rosive sublimate, by its being dissolved and evaporated in the 

 atmosphere of a hot-house, is not appreciable to the senses. 

 [See Chemical Combinations in the Atmosphere, sec. IV., for 

 detailed information on this subject.] 



Ventilation is necessary, then, not to enable plants to exercise 

 their respiratory functions, provided the atmospheric air is un- 

 mixed with accidental impurities, but to carry off noxious vapors 

 generated in the atmosphere of a glazed house, and to produce 

 dryness, or cold, or both. Thus it is evident that air is given 

 under many conditions, when it is not only unnecessary, but 

 injurious. 



When air is admitted, to produce cold in the house, the 

 external temperature must be lower than the atmosphere of the 

 house. This effect, however, cannot always be produced by 

 ventilation, as, in summer, if the houses be rightly managed, the 

 reverse effect will be produced, as the external air is not only 

 warmer but more drying in its nature than the air of the 

 house ought to be ; therefore its admission can only prove inju- 

 rious, rather than otherwise, as we shall afterwards show. On 

 the other hand, if the external air be cold, its admission will 



