536 



DETAILS OF CONSTRUCTION. 



the influence of red light than under any- 

 other. Thirdly, " light which has per- 

 meated blue media/' The rays thus sepa- 

 rated from the heat and the light rays, 

 which are regarded by Mr Hunt as a dis- 

 tinct principle, for which he has "proposed 

 the name of Actimism, have a power of 

 accelerating, in a remarkable degree, the 

 germination of seeds, and the growth of 

 the young plant. After a certain period, 

 varying with nearly every plant upon 

 which experiments have been made, these 

 rays became too stimulating, and growth 

 proceeds rapidly, without the necessary 

 strength. When this is perceived, the 

 removal of the plant into the yellow rays, 

 or, which is better, into light which has 

 permeated an emerald green glass, acce- 

 lerated the deposition of carbon, and the 

 consequent formation of woody fibre pro- 

 ceeds in a very perfect manner." 



Such, says Mr Hunt, are the conditions 

 and results of his experiments. So much 

 for the colour of glass. 



In regard to the charges brought 

 against British sheet glass, on account of 

 inequality in thickness, undulation of 

 surface, &c, we must admit that, for a 

 time after its being first brought into 

 use, there was great cause for complaint. 

 This is, however, now much less the case, 

 as the article is of a superior manufac- 

 ture. The evils here referred to arose 

 from the convex parts forming lenses, 

 by which the rays of the sun became 

 concentrated at a distance from the 

 under surface of the glass, proportioned to 

 the convexity and diameter of the part. 

 These defects were found to exist, to 

 a very serious extent, in the large con- 

 servatory of the Botanical Society of 

 London, although great care was taken 

 by the curator, Mr Marnock, who states 

 the following interesting particulars; and, 

 coming from such authority, they merit 

 especial attention : — 



" In many instances," he says, " the 

 concentration of the rays only occurs at 

 the floor of the house ; and making allow- 

 ance for the oblique direction of these 

 rays, the distance from the glass is in 

 many cases not short of 40 feet. Many 

 others, again, are the products of much 

 smaller convexities, and the rays from 

 these concentrate, of course, at various 

 distances in the neighbourhood of the 

 under surface of the glass. Thus, it will 



be readily understood, must the whole 

 interior of the glass house, glazed with 

 inferior sheet glass, consist of an atmo- 

 sphere filled with a multitude of concen- 

 trated rays, proportioned in number to 

 the undulations on the surface of the 

 glass, meeting and crossing one another 

 in every possible direction, from within 

 a few inches of the glass to the distance 

 which we have already stated of feet, 

 or more. It is also worthy of remark, 

 and it is important in a practical point 

 of view, that the larger — that is, the 

 broader the convex parts, the less injury 

 do they commit — for the two following 

 reasons : First, that when the convex part 

 or lens in the glass is several inches in 

 diameter, and not much raised, the rays 

 of the sun cannot meet through this 

 form but at a considerable distance from 

 the glass ; and, secondly, in proportion 

 as the concentration of the rays is distant 

 from the glass, so in like manner does 

 the focus, or point at which they meet, ac- 

 quire motion. Hence, except upon broad- 

 leaved plants— such as camellias, orange 

 trees, vines, pines, peaches, and similar 

 plants, upon which injury is inflicted, 

 owing to the breadth of the leaves and 

 the time required for the concentrating 

 point to travel over their surface — the evil 

 is seldom much felt ; and if the leaves are 

 small, these distant rays do no damage 

 whatever — that is, unless the glass be of 

 the very worst possible quality. 



" It will, however, be very properly 

 inferred from what we have stated, that 

 the matter is very different with the 

 smaller undulations, when the house is 

 used for forcing, and the leaves are 

 necessarily near the glass. In such 

 cases it is impossible that mischief should 

 not occur ; and this will be the case in 

 defiance of skill or care, however applied, 

 unless the best glass has been employed. 

 When horticultural erections are raised 

 for early forcing, and the British sheet 

 glass is used, it would seem prudent to 

 construct the trellis upon which the 

 plants are trained at a greater distance 

 from the glass than that at which it is 

 usually placed. It is also well to bear 

 in mind that wavy glass, with its surface 

 much undulated, refracts a portion of the 

 light, which, by smooth and superior 

 glass, would be transmitted into the 

 house for the benefit of the plants." 



