226 THE ATMOSPHEBE OP PLANT-HOUSES. 



40°, 20° of dryness are kept up in the interior, 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 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 annihi- 

 lated, 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 precaution 

 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 present be guarded against by diminishing the 

 interior heat in proportion. 



By materially lowering the temperature, Ave communicate a check 

 which is totally inconsistent with the welfare of tropical vegetation. 

 The chill which is instantaneously communicated to the glass by a fall 

 of rain or snow, and the consequent evaporation from its surface, must 

 also precipitate the internal vapour, and dry the included air to a very 

 considerable amount, and the effect should be closely watched. I do 

 not conceive that the diminution of light which would be occasioned 

 by the double panes would be sufficient to occasion any serious objec- 

 tion to the plan. The difference would not probably amount to as 

 much as that between hothouses with wooden rafters and lights, and 

 those constructed with iron bars. It might also possibly occasion a 

 greater expansion of the foliage ; for it is known that in houses with a 

 northern aspect the leaves grow to a larger size than in houses which 

 front the south. Nature thus makes an effort to counteract the defi- 

 ciency of light by increasing the surface upon which it is destined 

 to act. 



The present method of ventilating hothouses is also objectionable, 

 upon the same principles which I have been endeavouring to explain. 

 A communication is at once opened with the external air, while the 

 hot and vaporous atmosphere is allowed to escape at the roof; the con- 

 sequence is, that the dry external air rushes in with considerable 

 velocity, and, becoming heated in its course, rapidly abstracts the 

 moisture from the pots and foliage. This is the more dangerous, inas- 

 much as it acts with a rapidity proportioned in a very high degree to 

 its motion. I would suggest, as a matter of easy experiment, whether 

 great benefit might not arise from warming the air to a certain extent, 

 and making it traverse a wet surface before it is allowed to enter the 

 house. 



There is one practice universally adopted by gardeners, which is 

 confirmatory of these theoretical speculations, namely, that of planting 

 tender cuttings of plants in a hot-bed, and covering them with a double 

 glass. Experience has shown them that many kinds will not succeed 



