50 



CHAP. VIII. 

 ON THE ADMISSION OF AIR. 



The question of the admission of air, is one of some 

 importance. It is an opinion, which was I believe first 

 publicly brought forward by the late Mr. Knight, that 

 an influx of a large volume of the external atmosphere, to 

 the interior of forcing houses, is by no means requisite, 

 and is often the source of very serious evils. Were it for 

 no other reason, than that of avoiding the chilling in- 

 fluence of cold air on the tender tissue of plants growing 

 in a high temperature, I should feel inclined to support 

 such a view ; but when there are facts sufficiently abun- 

 dant, to prove, that plants do not themselves vitiate the 

 air of such structures to an extent sufficient to render it 

 unfit for their continued growth, or at least, that a suffi- 

 cient interchange is constantly going on, without opening 

 the sashes of a forcing house, the evidence appears to be 

 overwhelming ; and the necessity of continuing a practice 

 so fraught with danger, and so frequently attended with 

 disappointment, appears to be done away. 



The injury done to the tender foliage of plants in forcing 

 houses, by contact with cold air, results from the increased 

 capacity of air for moisture, as it become heated. When 

 cold air is admitted to these structures, it cannot contain 

 so great a quantity of aqueous matter, as it is capable of 

 taking up when it becomes warmed : this increase of tem- 

 perature, is soon in great measure, supplied to it, but 

 rarely is a sufficient quantity of moisture, at the same 

 time within its reach, to enable it to supply its increased 

 capacity for aqueous matter : the consequence is, that on 

 coming in contact with the foilage of the plants, which is 

 of a succulent nature, and contains a great proportion of 

 water, the warmed air continues to abstract a portion of 



