GLASS. Ill 



and familiar in regard to it, yet the evils seem to be passed over 

 as results which cannot be prevented. We can at this moment 

 point to houses standing side by side, in one of which it is 

 impossible to grow, and keep in health, any species of vegeta- 

 tion whatever, no matter how hardy the tissue of the foliage 

 may be, without shading the glass almost to opacity ; while, in 

 the other, plants with tender and delicate foliage stand compar- 

 atively uninjured. The cause is obvious : the glass with which 

 the one is glazed is full of waves and blotches, and altogether 

 of the worst description ; while that of the other, though not the 

 best, is yet of better quality. The poorer glass burns vegetation, 

 even when the incidental angle, between the impinging ray and 

 a perpendicular to the roof, is as much as 45. 



From what has been already said regarding the influence of 

 the different solar rays on vegetation, and, more especially, the 

 experiments made with regard to the Palm-house at Kew Gar- 

 dens, by which it has been found possible to manufacture glass 

 which is opaque to the scorching rays, without at the same time 

 obstructing the light, heat, and chemical rays which are essen- 

 tial to the development of plants, there can be no doubt that 

 the scorching of vegetation in hot-houses, which has long been 

 a serious drawback in exotic horticulture, can be prevented. 

 And when more extended experiments have been made, a good 

 material for glazing can undoubtedly be manufactured at a price 

 that will insure its universal adoption in horticultural structures. 

 It is to be earnestly desired that some of our enterprising manu- 

 facturers, a class so remarkable for their fertility of invention, 

 will take up the matter seriously, and supply us with the 

 material which exotic horticulture so much requires. 



2. Glazing. Common sash-glazing is generally performed 

 with a lap of from one to three fourths of an inch, and, by many, 

 with a full inch lap. This is a most objectionable method, as 

 the broader the lap the greater the quantity of water retained 

 in it by capillary attraction, and, consequently, the greater the 

 breakage of the glass ; for when the internal temperature falls, 

 and this water becomes frozen, the glass is certain to crack in 

 the direction of the bars. The lap should never be broader than 

 10* 



