218 



FIXED STRUCTURES FOR GROWING 



high as that of the house ought to be, openings may be made in every 

 direction at pleasure. In stoves the precaution of covering the openings 

 of the upper part of the roof, by which air is given, with wire netting, might 

 be taken, which, while it excludes wasps and flies in summer, would in 

 winter act like Jeffrey's Respu*ator, in abstracting the heat from the 

 heated air which escaped, and imparting it to the cold air which entered ; 

 or the double tube, recommended by Dr. Arnott in his Treatise oil Warming 

 and Ventilating^ might be adopted. The external au* may be heated in the 

 w^inter season before it is allowed to enter the house, by enclosing a part of 

 the pipes or smoke flues in a trunk or box, with a communication at the 

 lower part of one end with the open air, and at the upper part of the other 

 with the air of the house. So long as the pipes are kept at a temperature 

 considerably above that of the house, fresh air will flow in, and a corre- 

 sponding quantity will be displaced by the accidental crevices of the roof. 

 In hotbeds it is customary to leave openings for the escape of moist vapour 

 during the whole of the night ; this is generally done by raising the sashes 

 behind, but, as by this mode the steam from the dung is sometimes driven 

 in, some gardeners have a narrow opening in the upper part of the sash, with 

 a lid to fit to it, hmged along the upper edge. 



512. Light is one of the elements of culture as essential as heat (278). 

 When the object is merely to grow plants without fruiting them, the pro- 

 portion of glass may be small, provided it be pretty equally'' distributed over 

 the roof ; but when the object is to produce flowers and fruit, the proportion 

 of glass to the wood or metal of the roof ought to be greater. In nursery- 

 men's houses for growing plants, the ordinary size of the panes used to be 

 five inches by three inches, and they were made of the common crown glass, 

 which was very thin, a heavy duty being payable by the manufacturer on 

 glass which was estimated not by the weight but the measure. When, 

 however, the duty on glass was taken ofi^, manufacturers were enabled to 

 make glass of any size and thickness that was required ; and gardeners 

 wishing to give their plants as much light as possible, frequently used panes 

 of the British sheet glass, two or even three feet long, and a foot or fourteen 

 inches in width. Panes of this enormous size required to be of proportionate 

 thickness, and accordingly the sheet glass generally weighed from 18 oz. to 

 26 oz. the foot. As this glass was beautifully clear, the most beneficial 

 effects were expected from the free admission of light to the plants ; but 

 the result was not equal to what was anticipated. The surface of the thick 

 glass was found unequal when examined through a powerful microscope, 

 and every inequality was found to act as a lens in collecting the sun's rays, 

 and scorching the leaves of the plants. To obviate this inconvenience the 

 idea was suggested of using coloured glass, and green glass was used in the 

 large conservatory at Kew. It was not, however, found to answer, and a 

 kind of rough glass was introduced, which was found not only to diffiise 

 the light equally, without scorching the leaves, but to render shading 

 unnecessary. When this glass is used, the most convenient size for the 

 panes is eighteen inches by six inches or nine inches, weighing sixteen ounces 

 to a foot. As, however, the rough glass is very unsightly, some persons pre- 

 fer crown glass in squares, from nine inches to a foot square, which in glaz- 

 ing are made to lap over one another, from one-eighth to one-fourth of an 

 inch. In general one-eighth of an inch is quite sufficient ; as the broader the 

 laps, the greater is the quantity of water which they retain, and the more 



