126 



HOTHOUSE-BUILDING. 



are, for the reasons above stated, prefer- 

 able to iron." This opinion coincides 

 with that lately expressed by Professor 

 Lindley, and long entertained by our- 

 selves. Condensation goes on more rapidly 

 in metallic houses than in any other, and 

 more so in roofs of a flat pitch than in 

 those more upright. This is the case in 

 all houses, particularly in cold weather ; 

 but it takes place more especially in 

 those having metallic roofs, as, in addition 

 to the cold surface of the glass, which 

 will be nearly the same in both, we have 

 a considerably increased amount of con- 

 densation from the metal also. The ex- 

 cellent paper by the late Professor Daniel, 

 published in "The Transactions of the 

 Horticultural Society," brings this and 

 its consequences before us in a very clear 

 light : by reason of the abstraction of the 

 necessary humidity which ought to exist 

 in the atmosphere of a properly regulated 

 hothouse, by its coming in contact with, 

 and condensing upon the cold glass and 

 still colder metal, the air within becomes 

 so completely dried up as to be totally 

 unfit for vegetation. The leaf of a vine, 

 for example, in its young and growing 

 state, requires an atmosphere, the degree 

 of saturation of which is 800° nearly ; 

 now, if the saturation falls 300° below 

 this, the leaves will be dried up and 

 perish. The authority above quoted 

 says, "The heat of the glass of a hot- 

 house at night cannot exceed the mean of 

 the external and internal air; and, taking 

 these at 8° and 40° respectively, 20° of 

 dryness are kept up in the interior, or a 

 degree of saturation not exceeding 528°. 

 To this, in a clear night, we may add at 

 least 6° for the effects of radiation, to 

 which the glass is partially exposed, 

 which would reduce the saturation to 

 434° ; and this is a degree of drought 

 which must be nearly destructive. When 

 it is considered that a temperature at 

 night of 20° is no very unfrequent occur- 

 rence in this country, the saturation of 

 the air may, upon such occasions, fall to 

 120°. That is to say," Dr Lindley ob- 

 serves, when remarking on the above 

 quotation, "instead of the moisture in 

 the atmosphere surrounding vine leaves 

 amounting to 7 or 8 parts in 10, which 

 is what they require, it may not amount 

 to more than 1^ in 10, which is fatal to 

 them." 



The doctrine laid down by Professor 

 Daniel, and subscribed to by Professor 

 Lindley, involves the inference that the 

 humidity abstracted from the atmosphere 

 of the house, by the latter being brought 

 into contact with the cold surface of glass, 

 is decomposed or entirely lost. Be this 

 as it may, one thing is quite certain, that 

 the excessive conduction of both heat and 

 cold, together with the loss of humidity, 

 in whatever way this ensues, have been 

 regarded by most cultivators as one of 

 the great evils chargeable to metallic 

 hothouses. 



The advantages and disadvantages of 

 metallic hothouses are thus stated by Mr 

 Loudon, who, it should be observed, had 

 a strong feeling in their favour. He 

 says, " The material of fixed roofs is gene- 

 rally iron, as being less bulky in propor- 

 tion to the strength required, most 

 durable, and admitting, in the case of 

 curvilinear roofs, a curvature to be 

 formed at less expense than it could be 

 of timber. In those roofs, in general, no 

 other bars or opaque bearers are required 

 than those for securing the glass, and 

 hence their simplicity and unity with re- 

 gard to component parts, and the equal 

 degree of transparency in every part 

 of the surface." The same authority 

 goes on to say, " In general, it may be 

 observed, that till lately gardeners had a 

 prejudice against metallic roofs. We shall 

 here, as briefly as possible, enumerate 

 these objections, which are expense, rust, 

 breakage of glass, abduction of heat, 

 and attraction of electricity." In regard 

 to expense, he says, " Metallic houses 

 are, in general, more expensive than 

 wooden ones; but they admit more light, 

 and are more durable and elegant." To 

 this we answer, that the difference in cost 

 is from one-third to one-half, and often 

 more; the difference of light admitted, 

 especially now that large glass is within 

 our reach, is so trifling as to be practi- 

 cally considered of no account. That 

 they are more durable would, at first 

 sight, appear feasible enough ; but ex- 

 perience has not proved this to be the 

 case; indeed, the very reverse seems 

 established, for we know of wooden houses 

 that have existed for a century, but of no 

 metallic house that has stood half the 

 time. That they are more elegant we do 

 not deny. 



