THE ATMOSPHERE OF PLANT-HOUSES. 225 



exigencies of the plant when exposed to drought ; and the quantity 

 Avhich is known to vary in the hothouse is no doubt connected with the 

 state of moisture of the atmosphere. 



These considerations must be sufficient, I imagine, to place in a 

 strong light the necessity of a strict attention to the atmosphere of 

 vapour in our artificial climates, and to enforce as absolute an imitation 

 as possible of the example of nature. The means of effecting this is 

 the next object of our inquiry. 



Tropical plants in stoves require to be watered at the root with 

 great caution ; and it is impossible that a sufficient supply of moisture 

 can be kept up from this source alone. There can, however, be no 

 difficulty in keeping the floor of the house and flues constantly wet ; 

 and an atmosphere of great elasticity may thus be maintained in a way 

 perfectly analogous to natural process. Where steam is employed as 

 the means of communicating heat, an occasional injection of it into the 

 air may also be had recourse to ; but this method would require muoh 

 attention on the part of the superintendent, whereas the first cannot 

 easily be carried to excess. 



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

 would also be detrimental to the health of the plants, for it is abso- 

 lutely necessary that the process of transpiration should proceed ; but 

 there is no danger that the high temperature of the hothouse should 

 ever attain the point of saturation by spontaneous 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 deposition is governed by the temperature of the glass with 

 which it is in contact. In a well-ventilated hothouse, 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 

 ten or fifteen degrees, but the degree of saturation 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 esta- 

 blished, 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 pro- 

 ceeds 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 hothouse the same degree 

 of heat at night as during the day, the plants become scorched. From 

 what has been premised, 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 hothouse at night does not probably exceed 

 the mean of the external and internal air ; and taking these at 80 J and 



