2088. Objections to metallic roofs. In general, it may be observed, that, till lately, gardeners had a prejudice against metallic roofs. We shall here, as briefly as possible, enumerate their objections; which are, expense, rust, breakage of glass, abduction of heat, and attraction of electricity. Expense. Metallic houses are, in general, rather more expensive than wooden ones; but they admit more light, and are more durable and elegant. Rust. That all ordinary metals are liable to rust is undeniable. This objection cannot be got rid of. The reply is, balance against it the advantages of light and durability; and take into consideration, that careful painting will, in a great degree, prevent it. Knight observes, if one third of the sum requisite to keep a wooden roof properly painted be expended upon an iron roof, no injury will ever be sustained from the liability of that to suffer from rust. (Hort. Trans., vol. v. p. 231.) Breakage of glass. This is altogether denied, as respects cast or wrought iron, at least; and if applicable at all, can only be so to copper or compound metallic roofs, where weakness produces a bending of the sash; or where corrosion or unequal expansion of improper mixtures of metals, as iron cased with copper, occasions a twisting of the bar. Cast-iron or solid wrought-iron frames have never been known to occasion the breakage of more glass than wood. The expansibility of copper is greater than that of brass, and that of brass greater than the expansion of iron, in the proportion of 95, 83, 60. (Young's Lect.) Consequently, copper is above one third part more likely to break glass than iron; but, when it is considered that a rod of copper expands only one hundred thousandth part of its length with every degree of heat; and that iron only expands the one hundred and sixty-six thousand six hundred and-sixty-sixth part, the practical effects of our climate on these metals can never amount to a sum equal to the breakage of glass. Abduction of heat. The power of metals to conduct heat is an objection which, like those of rusting and additional expenses cannot be denied. The reply is, the smaller the bars, the less their power of conducting; and a thick coat of paint, and the covering of half the bar by the putty requisite to retain the glass, also lessens this power: it is added, heat may be supplied by art; but solar light, the grand advantage gained by metallie bars, cannot, by any human means, be supplied otherwise than by the transparency of the roof. Attraction of electricity. To this objection it is replied, that if metallic hothouses attract electricity, they also conduct it to the ground, so that it cannot do any harm. Also, that no instance can be produced of iron hothouses having been injured by the effects of this fluid.