312 SCIENCE OF GARDENING. p ARr If. 



where the habitation is to be erected ; or for such indigenous or acclimated plants as it 

 is desired to force or excite into a state of vegetation, or accelerate their maturation 

 at extraordinary seasons. The former description are generally denominated green- 

 houses or botanic stoves, in which the object is to imitate the native climate and 

 sod of the plants cultivated ; the latter comprehend forcing-houses and culinary stoves, 

 in which the object is, in the first case, to form an exciting climate and soil, on general 

 principles ; and in the second, to imitate particular climates. The chief agents of ve- 

 getable life and growth are heat, light, air, soil, and water ; and the merit of artificial 

 climates consists in the perfection with which these are supplied. 



1592. Such heat as is required in addition to that of the sun is most generally produced 

 by the ignition of carbonaceous materials, which heat the air of the house, either directly 

 when hot embers of wood are left in a furnace or stove, placed within the house, as in 

 Sweden and Russia ; mediately, as when smoke and heated air, from, or passing through 

 ignited fuel, is made to circulate in flues ; or indirectly, when ignited fuel is applied *to 

 boil water, and the hot vapor, or the water itself, is impelled through tubes of metal or 

 other conductors, and either to heat the air. of the house at once, as in most cases, or to 

 heat masses of brick-work, sand, gravel, rubble, or earth, tan, or even water, (Hort. 

 Trans, vol. iii.) which materials may afterwards give out the heat so acquired slowly to 

 the atmosphere of the house. But heat is also occasionally supplied from fermenting 

 vegetable substances, as dung, tan, leaves, weeds, &c. applied either beneath or around 

 the whole or a part of the house, or placed in a body within it. 



1593. In "particular situations heat may be obtained from anomalous sources, as in Iceland, 

 Toeplitz, and Matlock, from hot springs ; and perhaps in some cases, especially in coal 

 districts, from a basement composed of certain compounds of sulphur and iron, &c. 

 Dr. Anderson ( Treatise on the Patent Hot-house,) proposed to preserve the superfluous 

 heat generated by the sun in clear days, and to retain it in reservoirs placed under, 

 above, or at one side of the house, re-admitting it as wanted to keep up the temperature ; 

 but the plan, though ingenious and philosophical, required too much nicety of execution, 

 and the clear days in this country are too few to admit of adopting it as a substitute for 

 heating by ignition. Heat must not only be produced in hot-houses, but its waste avoided, 

 by forming as large a portion of the cover as possible of materials through which it 

 escapes with difficulty, as far as this is consistent with other objects. Hence, in certain 

 classes of houses, the side to the north is formed of opaque and non-conducting 

 materials. 



1594. Light is admitted by constructing the roof, or cover, of transparent matter, as 

 oiled paper, talc, or glass, (the last being found much the best material,) joined to as small 

 a proportion of opaque substances, as timber or metal, as is found consistent with the 

 strength requisite to bear the weight of the glass, resist the accidents of weather, &c. 

 All plants require perpendicular light, but some, as many succulents and others, which 

 throw out, or are allowed to radiate their branches on all sides, require the direct influ- 

 ence of light on all sides ; others naturally, as creepers or climbers, or artificially, when 

 rendered creepers or climbers, by the art of training on walls or trellises, require direct 

 light on one side only ; and hence it is, that for certain purposes of culture, hot-houses 

 answer perfectly well when the transparent covering forms only a segment of their 

 transverse section, provided that segment meets the sun's rays at a large angle the greater 

 portion of the growing season. This, of course, is subject to limitations and variations 

 according to circumstances, and has given rise to a great variety in the external forms 

 of hot-houses, and the angles of their roofs. It decides, however, the necessity of 

 placing all houses whose envelope is not entirely transparent, with their glazed side to 

 the south. 



1595. The introduction and management of light is the most important point to attend to in 

 the construction of hot-houses. Every gardener knows, that plants will not only not thrive 

 without abundance of light, but will not thrive unless they receive its direct influence by 

 being placed near or at no great distance from the glass. The cause of this last fact 

 has never been satisfactorily explained. (Sowerby on Light and Colors, 1816.) It seems 

 probable, that the glass acting in some degree like the triangular prism, partially de- 

 composes or deranges the order of the rays. It is an important fact also, that light in 

 nature is always accompanied by heat ; and, therefore, it should not only be an object to 

 admit the sun's direct rays in clear weather, when he is visible, but even when the rays 

 are refracted and deranged by clouds and vapors, when he is invisible. 



1596. The theory of the transmission of light through transparent bodies, is derived from 

 a well known law in optics, that the influence of the sun's rays on any surface, both in 

 respect to light and heat, is directly as the sine of the sun's altitude, or in other words, 

 directly as his perpendicularity to that surface. If the surface is transparent, the num- 

 ber of rays which pass through the substance is governed by the same laws. Thus, if 

 1000 rays fall perpendicularlv upon a surface of the best crown-glass, the whole will 

 pass through, excepting about a fortieth part, which the impurities of even the finest 



