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LVII. On the Ventilation of Forcing-houses. By Thomas 

 Andrew Knight, Esq. F. R. S. Sfc. President. 



Read May 7, 1816 



In a Memoir, which I had two years ago the honour to 

 address to the Horticultural Society,* I stated an opinion that 

 the gardener often erred in the application of heat, by treat- 

 ing his plants as he would wish to be himself treated, and 

 consequently by keeping them much too warm during the 

 night. Experiments, made previously and subsequently to 

 that period, have satisfied me that he as often, and as widely 

 errs, by too freely admitting the external air during the day, 

 particularly in bright weather. Plants generally grow best, 

 and fruits swell most rapidly, in a warm and moist atmos- 

 phere ; and change of air is, to a very limited extent, neces- 

 sary or beneficial. The mature leaves of plants, and, ac- 

 cording to Saus sure, the green fruits, (grapes at least,) when 

 exposed to the influence of fight, take up carbon from the 

 surrounding air, whilst the same substance is given out by 

 every other part of the plant ; so that the purity of air when 

 confined in close vessels has often been found little changed 

 at the end of two or three days by the growth of plants in it. 

 But even if plants required as pure air as hot-blooded 

 animals, the buoyancy of the heated air, in every forcing- 

 house, would occasion it to escape, and change as rapidly, 

 and indeed much more rapidly, than would be necessary. 



* See page 130 of this Volume. 



