EFFECTS OF VENTILATION. 267 



the same effects are produced by the excessive evaporation as 

 by the currents of cold air, as will be afterwards shown. 



5. Ventilation is also required, in winter, in pits and frames 

 where soft and succulent plants are grown, especially in pits and 

 frames warmed with fermenting materials. In this case, much 

 care and caution are necessary ; the object here being to carry off 

 the superfluous moisture, in order that the succulent tissue of 

 the plants may not absorb more aqueous matter than they can 

 decompose and assimilate. Although these kinds of plants will 

 bear a high degree of atmospherical moisture in summer, when 

 the days are long and the sun bright, and when, consequently, 

 all their digestive energies are in full activity, yet they are 

 by no means able to endure the same amount in the dark, short 

 days of winter, when their powers of decomposition, or diges- 

 tion, are comparatively feeble. 



6. The thermometric changes are by no means satisfactory 

 guides for regulating the admission of air in hot-houses, as the 

 effect required by the indications of the thermometer may be 

 produced without resorting to the admission of air. In hot- 

 houses, we have full control over the state of the atmosphere, 

 both as regards its moisture and temperature ; and the means 

 of exercising this power ought to be known and familiar to 

 every gardener. But there are many circumstances which 

 ought to be duly considered in the exercise of this power, and 

 some unsuspected results arise from the unlimited use and exer- 

 cise of it ; and, as has been already said, by far the greater 

 number of gardeners attach too much importance to the mere 

 opening and shutting of sashes, w^indows, etc., without duly 

 studying the rationale of the practice. We will show that the 

 practical effects of ventilation are not only different from what 

 many suppose, but are actually injurious. 



During winter we are in the habit of raising the temperature 



of our hot-houses, by artificial heat, to 45 or 50° ; then, for six 



or seven hours during the day, we open the lights and admit a 



large quantity of cold air. This is also a stumbling-block, on 



which a great many gardeners fall ; for it is not solely to the 

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