MATERIALS OF CONSTRUCTION. 103 



in the proportion of 95 to 60, therefore copper is above one third 

 more likely to break glass than iron. But when it is considered 

 that a rod of copper expands only x^uVtrir P^^^ of its length with 

 every degree of heat, and that iron only expands y^uVs^-B" P^rt, 

 the practical effects of even the hottest portion of our climate 

 on these metals can never amount to a sum equal to the expan- 

 sion required for the breakage of glass. 



The second objection which we have mentioned is also unde- 

 niable. All metals are liable to rust; but painting easily rids us 

 of this objection, at least it will so far prevent it as to form 

 hardly any objection. 



The power of metals to conduct heat is an objection which, 

 like the others, cannot be denied, but m.ay be partially obviated. 

 The abduction of heat, like the expansibility of metallic roofs, 

 is very little felt in using them ; the smaller the bars, the less 

 their power of conduction. The paint, also, and the putty used 

 to retain the glass, obviate this objection. Heat may be supplied 

 by art, but light, the grand advantage gained by metallic bars, 

 cannot, by any human means, be supplied but by transparency 

 of roof. 



The objection raised on the ground of attraction of electricity, 

 is easily answered. If metallic hot-houses and conservatories 

 attract electricity, they also conduct it to the ground, so that it 

 can do them no harm. What is corroborative of this position 

 is the fact, that no instance has come under our knowledge of 

 iron hot-houses having been injured by the electric fluid. 



The objection regarding the expense of iron hot-houses, has 

 been sufficiently refuted in England, and we have observed, 

 with pleasure, a refutation of the same objection, by an enter- 

 prising gentleman of Cincinnati, who has lately erected an iron- 

 roofed vinery. Mr. Resorr has given a cut, and description of 

 this house, in the " Horticulturist " for Sept. 1849, p. 117. This 

 is the only substantial account we have seen of the comparative 

 cost of iron and wood roofs. This gentleman, who is in the 

 foundery business, has every opportunity of knowing the accu- 

 rate cost of such a house, and plainly states, " that those wish- 

 ing to build a good, substantial house, can do it, and make the 

 roof of iron, as cheaply as of \yood, the other parts costing th» 



