GENERAL PRINCIPLES. 



127 



As regards rust, Mr Loudon observes, 

 "That all ordinary metals are liable to 

 rust is undeniable. This objection can- 

 not be got rid of. The reply is, take into 

 consideration the advantages of light 

 and durability, and that careful painting 

 will in a great degree prevent it. Knight 

 observes, that if one-third of the sum re- 

 quisite to keep a wooden roof properly 

 painted be expended upon an iron roof, 

 no injury will be sustained from the 

 liability of that to suffer from rust." 

 Neither of these gentlemen, in their en- 

 thusiasm to recommend iron roofs, ap- 

 pears to have taken into consideration 

 that the incipient seeds of the disease of 

 corrosion have been implanted before the 

 roofs are painted at all ; and let them be 

 painted as often as they may, the disease 

 is still progressing, in a way somewhat 

 analogous to the decay of unseasoned 

 timber; and so far as relates to the latter, 

 paint is rather injurious than beneficial. 

 Galvanised iron was not in use in those 

 days ; and, after all, even this process may 

 turn out as little to the preservation of 

 the metal as painting could. Were iron 

 secured against the effects of corrosion 

 until it was erected in a roof, glazed, and 

 painted, then, and not till then, would 

 we subscribe to the above opinion. We, 

 however, believe that means may yet be 

 discovered to prevent the oxidation of iron, 

 and all other metallic bodies whatever. 



The breakage of glass by contraction 

 and expansion in iron roofs, and the 

 yielding-in of copper and other over light 

 ones, has been admitted by most who 

 have had the superintendence of them. 

 This is denied by the advocates for them, 

 more especially as regards those of wrought 

 or cast iron ; though it is partially ad- 

 mitted by them in the case of copper, or 

 compound metallic roofs, "where weak- 

 ness produces a bending in 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." This 

 opinion may be very well in the study, 

 but it is not so good in the field. " The 

 expansibility of copper is greater than 

 that of brass, and that of brass greater 

 than that of iron, in the proportion of 

 95, 89, 60 ; consequently, copper is about 



one-third more likely to break glass than 

 iron. But when it is considered that a 

 rod of copper expands only 100,000th 

 part of its length with every degree of 

 heat, and that iron only expands the 

 165,666th part, the practical effects of 

 our climate on these metals can never 

 amount to a sum equal to the breakage 

 of glass." Glass neither expands or con- 

 tracts, neither is it elastic ; the expansion, 

 therefore , taken even at the low calcula- 

 tion above stated, is quite sufficient to 

 effect breakage to as great an extent as 

 we or any other have asserted, more espe- 

 cially if light glazed. " The power of 

 metals to conduct heat is an objection 

 which, like those of rusting and additional 

 expense, cannot be denied. The reply is, 

 the smaller the bars, the less their power 

 of conduction ; 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 can 

 be supplied by art ; but solar light, the 

 grand advantage gained by metallic bars, 

 cannot by any human means be supplied 

 otherwise than by the transparency of 

 the roof." This is all very well, but may 

 not too much light be admitted to plants 

 in a strictly artificial climate 1 else why 

 do we use obscured glass and shading? 

 Notwithstanding these objections, we are 

 quite aware that such roofs must be 

 used in such structures as the large 

 palm stove lately erected at Kew, the 

 conservatories in the Regent's Park, Sion 

 House, &c; and also that the large house 

 at Chatsworth, and Messrs Loddige's palm 

 stove, would have been much more elegant 

 in exterior appearance had they been so 

 constructed. To those who build such 

 lofty houses, we say, by all means employ 

 iron, if you can afford the cost, and take 

 the necessary precautions to ventilate 

 them properly. After all that has been 

 said against metallic houses of great 

 height, so far as the conduction of heat 

 is concerned, we believe the most im- 

 portant thing has been next to neglected, 

 — namely, sufficient means of ventilation, 

 more especially at or near the top. 



We still maintain that properly-con- 

 structed wooden houses, without rafters or 

 framed sash es, built on the ridge-and-furrow 

 principle, with metallic vallej'S to carry 

 off the water, are much less expensive in 

 erection; equally, if not more durable 



