ON THE CLIMATE OF HOTHOUSES. 273 



must be nearly destructive. It will be allowed tbat 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 con- 

 sequently tend to prevent the dissipation of the heat. But no 

 means would of course bo so effective as double glass, including 

 a stratum of air ; indeed, such a precaution in winter seems al- 

 most 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 sat- 

 uration 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, w*e 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 evapora- 

 tion from its surface, must also precipitate the internal vapour 

 and dry the included air to a very considerable amount, and the 

 effect shoidd 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 objection to the 

 plan. The difference would not probably amount to as much as 

 that between hot-houses with wooden rafters and lights, and those 

 constructed with curvilinear iron bars, two of which have been 

 erected in the garden of the Horticultural Society. 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 deficiency of light, by increas- 

 ing the surface upon which it is destined to act. 



The present method of ventilating hot-houses is also objection- 

 able, upon the same principles which I have been endeavouring 

 to explain. A communication is at once opened with the exter- 

 nal air, while the hot and vaporous atmosphere is allowed to es- 

 cape at the roof; the consequence is, that the dry external air 

 rushes in witli considerable velocity, and becoming heated in its 

 course, rapidly abstracts the moisture from the pots and foliage. 

 This is the more dangerous inasmuch as it acts with a rapidity 



