272 ON THE CLIMATE OF HOTHOUSES. 



superintendant, whereas the first cannot easily be carried to 

 excess. 



It is true that damp air, or floating moisture of long continu- 

 ance, would also be detrimental to the health of the plants, for 

 it is absolutely necessary that the process of transpiration should 

 proceed ; but their is no danger that the high temperature of the 

 hot-house should ever attain the point of saturation by sponta- 

 neous evaporation. The temperature of the external air will 

 always keep down the force of the vapour ; for as in the natural 

 atmosphere the dew-point at the surface of the earth is regulated 

 by the cold of the upper regions, so in a house the point of de- 

 position is governed by the temperature of the glass with which 

 it is in contact. In a well ventilated hot-house, by watering the 

 floor in summer, we may bring the dew-point within four or 

 five degrees of the temperature of the air, and the glass will be 

 perfectly free from moisture ; by closing the ventilators, we shall 

 probably raise the heat 10 or 15 degrees, but the degree of satu- 

 ration will remain nearly the same, and a copious dew will quickly 

 form upon the glass, and will shortly run down in streams. A 

 process of distillation is thus established, which prevents the 

 vapour from attaining the full elasticity of the temperature. 



The action is beneficial within certain limits, and at particular 

 seasons of the year ; but when the external air is very cold, or 

 radiation proceeds very rapidly, it may become excessive and 

 prejudicial. It is a well known fact, but one which, I believe, 

 has never yet been properly explained, that by attempting to 

 keep up in a hot-house the same degree of heat at night as during 

 the day, the plants become scorched. From what has been pre- 

 mised, it will be evident that this is owing to the low temperature 

 of the glass, and the consequent low dew-point in the house, 

 which occasions a degree of dryness which quickly exhausts the 

 juices. 



Much of this evil might be prevented by such simple and 

 cheap means as an external covering of mats or canvass. 



The heat of the glass of a hot-house at night, does not prob- 

 ably exceed the mean of the external and internal air; and ta- 

 king these at 80° and 40°, 20° of dryness are kept up in the in- 

 terior, or a degree of saturation not exceeding 528°. To this in 

 a clear night, we may add at least 6° for the effects of radiation, 

 to which the glass is particularly exposed, which would reduce 

 tbe saturation to 434°, and this is a degree of drought which 



