82 ATMOSPHERIC MOISTURE CONSIDERED. 



method would require much attention on the part of the superintendant, 

 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 absolutely necessary that the process of 

 transpiration should proceed ; but there is no danger that the high tem- 

 perature 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 established, 

 which prevents the vapour from attaining the full elasticity of the tempera- 

 ture. This 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 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 canvas; or by still slower conductors of heat, such as straw mats, or 

 " thatched hurdles ;" the latter, from the direction of the straws, throwing 

 off the rain, and, from their tubular construction, retaining a large proportion 

 of stagnated air, and hence forming an excellent non-conductor. 



261. *' The heat of the glass of a hothouse at night cannot exceed the mean 

 of the external and internal air, and taking these at 80 and 40, 20 of 

 dryness are kept up in the interior, or a degree of saturation not exceeding 

 628. To this in a clear night we may add at least 6 for the effects of radia- 

 tion, to which the glass is particularly exposed, which would reduce the 

 saturation to 434, and this is a degree of drought which must be nearly 

 destructive. It will be allowed that the case which I have selected is by no 

 means extreme, and it is one which is liable to occur even in the summer 

 months. Now by an external covering of mats, &c., the effects of radiation 

 would be at once annihilated, and a thin stratum of air would be kept in 

 contact with the glass, which would become warmed, and consequently tend 

 to prevent the dissipation of the heat. But no means would of course be so 

 effective as double glass, including a stratum of air. Indeed, such a precau- 

 tion in winter seems almost essential to any great degree of perfection in this 

 branch of Horticulture. When it is considered, that a temperature at night 

 of 20 is no very unfrequent occurrence in this country, the saturation of the 

 air may, upon such occasions, fall to 120; and such an evil can only at pre- 

 sent be guarded against by diminishing the interior heat in proportion. But 

 by materially lowering the temperature, we communicate a check which is 



