2SS MANAGEMENT OF THE ATMOSPHERE. 



respect to this practice, it cannot be denied that a risk is in- 

 curred, and frequently an injury sustained, when cold air comes 

 in contact with the active organs of tender plants. And, there- 

 fore, if no other advantage be gained from the practice than the 

 regulation of the temperature, then, except in cases where the 

 heat is increased by the influence of the sun, and therefore 

 uncontrollable, it would be a much wiser practice to apply a less 

 amount of heat by artificial means, thus rendering it less neces- 

 sary to allow the superabundant portion to escape, and conse- 

 quently exposing the plants in a less degree to the risk to 

 which we have alluded. 



4. Even in those cases in which it is really necessary to 

 have recourse to the practice of admitting air, much injury will 

 be sustained, though it may not be apparent at the time, by 

 admitting it in a rash and improper manner. It should be con- 

 trived so that the change to be effected may be brought about 

 gradually, and the cold and heated volumes should be made to 

 intermingle regularly together, and in a Avay that the internal 

 volume will be equally affected by it. Thus, if it be desirable 

 to admit a quantity of air equivalent to the reduction of 20° 

 of temperature, then the first consideration ought to be the 

 external temperature ; and the apertures of admission ought to 

 be regulated according to the calculations given at pp. 164 and 

 165, and in such a manner that the volume of air within the 

 house will not be deteriorated thereby, nor deprived of those 

 gases which are essential to vegetable existence. 



Secondly. How does the external air act upon the internal at- 

 mosphere, when so admitted ? This portion of our subject is of 

 more difficult solution, and requires a closer investigation, inas- 

 much as it is influenced by various causes, such as the form of 

 the structure, the method of admission, and the material of 

 which the interior part of the house is composed ; for example, 

 a house presenting a large surface of glass to the morning sun 

 requires to be sooner ventilated than one whose largest glass 

 •surface has a western aspect, and a small quantity of air admit- 

 ted early in the morning will keep the tem|7erature down for a 



