GLASS. The quality and thickness of Glass are important considerations in the construction of houses for horticultural purposes. Since the value of light for plants has been more fully recognised, and Glass of good quality has become much cheaper, improvements have been generally made which entirely supersede the old system of inserting very small panes. Various sorts of Glass have been tried at different times, but none is found to equal good sheets of not less than 21oz. or 24oz. to the square foot, where the panes are intended to be large. It is not advisable to have the latter more than 3ft. long for any plant structure, on account of their weight and the expense of repairs, should any breakage occur. Opque corrugated Sheet-glass, and rough or unpolished Plate-glass, have each been tried for various plant houses, but have been found unsuitable on account of admitting insufficient light in dull weather, and also as not affording the requisite shade for tender plants in summer without additional covering being applied. Green-tinted Glass is sometimes used where a subdued light is desirable, such as a house devoted to ferns. This does not prevent the necessity of giving other shade in addition on bright summer days. Excepting for frame sashes or small panes in houses, Glass that only weighs 16oz. to the square foot should not be used for glazing any framework that is exposed to hail or snow storms and other rough weather. Curvilinear roofs require specially bent Glass for certain parts, which, however, costs nearly double the price of the ordinary kind, and consequently renders repairs to such structures considerably more expensive. If Glass is of bad quality, the destruction, by burning, of the tender foliage of plants beneath is almost certain. This is caused by numerous defects in the Glass acting as foci.