On the Ventilation of Forcing-houses. 225 



It may be objected that plants do not thrive, and that the 

 skins of Grapes are thick, and other fruits without flavour, 

 in crowded forcing-houses ; but in these it is probably light, 

 rather than a more rapid change of air, that is wanting ; for 

 in a forcing-house, which I have long devoted almost ex- 

 clusively to experiments, I employ very little fire-heat, and 

 never give air, till my Grapes are nearly ripe, in the hottest 

 and brightest weather, further than is just necessary to pre- 

 vent the leaves being destroyed by excess of heat. Yet this 

 mode of treatment does not at all lessen the flavour of the 

 fruit, nor render the skins of the Grapes thick ; on the con- 

 trary, their skins are always most remarkably thin, and very 

 similar to those of Grapes which have ripened in the open 

 air. It is always my wish to see the temperature of this 

 house, in the middle of every bright day in summer, as high 

 as 90° ; and after the leaves of the plants have become dry, I 

 do not object to ten or fifteen degrees higher. In the fol- 

 lowing night the temperature sometimes falls as low as 50° ; 

 and so far am I from thinking such change of temperature 

 injurious, I am well satisfied that it is generally beneficial. 



Plants, it is true, tlirive well, and many species of fruits 

 acquire their greatest state of perfection in some situations 

 within the Tropics, where the temperature, in the shade, does 

 not vary in the day and night more than seven or eight de- 

 grees : but in these climates the plant is exposed during the 

 day to the full blaze of a tropical sun, and early in the night 

 it is regularly drenched with heavy wetting dews ; and con- 

 sequently it is very differently circumstanced in the day and 

 in the night, though the temperature of the air in the shade 

 at both periods may be very nearly the same. If the ther- 



