GLASS AND GLAZING. 



535 



is curvilinear, and these immense plates 

 of glass are bent to the necessary curve. 

 There is very little interruption to the 

 light, as none of the framework, which is 

 very slight, is nearer than 4 feet. The 

 light within is as clear as in the open 

 air, while the glare of the sun's rays is 

 intercepted so that no bad effects are 

 experienced by scorching. 



At the time the large tropical conser- 

 vatory at Kew was building, the glass 

 question was then engrossing the atten- 

 tion of all concerned in hothouse build- 

 ing, and scientific men began to turn 

 their attention so much to the subject 

 that the glass for this very structure 

 formed the text of a very interesting 

 paper read by R. Hunt, Esq., before the 

 British Association. And soon after- 

 wards we find that the Commissioners 

 of Woods and Forests, at the sugges- 

 tion of Sir William Jackson Hooker 

 and Dr Lindley, applied to that gentle- 

 man to ascertain " if it would be possible 

 to cut off these scorching rays by the 

 use of a tinted glass which would not be 

 obj ectionable in its appearance." After an 

 elaborate series of experiments, Mr Hunt 

 determined that a glass, coloured' of a 

 pale yellow-green, by means of oxide of 

 copper, gave the most satisfactory results. 

 The absence of the oxide of manganese, 

 commonly employed in the manufacture 

 of sheet glass, is insisted on — it having 

 been found that glass, into the composi- 

 tion of which manganese enters, will, 

 after exposure for some time to intense 

 sunlight, assume a pinky hue, and any 

 tint of this character would completely 

 destroy the peculiar properties for which 

 the glass is chosen. In fact, the experi- 

 ments of Mr Hunt do little more than 

 lead us back to the days when good 

 crown glass of a greenish hue was in 

 high repute for hothouse roofs. In the 

 experiments made on radiant heat, some 

 years ago, by Melloni, he discovered that 

 the colorific rays were nearly all obstruct- 

 ed by the green Italian glass used in his 

 country, for almost all ordinary purposes. 

 From all this, it would appear that glass 

 having a greenish tinge is the most 

 suitable for hothouse purposes, so far as 

 mere colour is concerned. On a subject 

 of so much importance as this— and we 

 hold the colour of glass to be so — other 

 opinions should be brought to bear on 



the matter also. The theory laid down by 

 DrDaubeny, in the "Philosophical Trans- 

 actions" for 1836, and corroborated by 

 M. Decaisne, is worthy of attentive 

 perusal — as are the experiments made 

 by Dr Horner of Hull, who recommends 

 violet-coloured glass, as not only affording 

 a partial shade, but as transmitting a 

 light which possesses a subtle action in 

 exciting vegetation, and proving, as he 

 says, in all respects an admirable auxi- 

 liary to the artificial heat and moisture 

 necessarily employed in culture. The 

 best paper, however, we have read on 

 this subject is that by R. Hunt, Esq., 

 already referred to, and published at 

 length in " The Gardeners' Chronicle," to 

 which we refer our readers. 



From this paper it appears that, by 

 the use of red, blue, and yellow glasses, of 

 peculiar kinds, we may so far alter the 

 natural condition of the plants as to free 

 them from one or more influences, whilst 

 the third is brought more decidedly into 

 action. Mr Hunt's experiments have led 

 him to conclude, that " red, blue, and 

 yellow media are suitable to all the 

 conditions necessary for producing the 

 following results : " First, " light which 

 has permeated yellow media " was found, 

 in almost all cases, to have the effect of 

 preventing the germination of seeds; and 

 in the few cases forming this exception, 

 the young plants soon perished. Mr 

 Hunt concludes that this is owing more 

 to the action of the heat rays than to 

 the light. He conjectures, however, that 

 although the luminous rays may be inju- 

 rious to vegetation, (which, no doubt, 

 they are,) and the early stages of vegeta- 

 tion, they are essential in after growth, 

 particularly in forming woody fibre. 

 Secondly, " light which had permeated red 

 media " is not unfavourable to germina- 

 tion, if the seeds be kept sufficiently 

 moist, to make up for the increased 

 evaporation. The plants, however, assume 

 a sickly appearance, and become partially 

 etiolated, showing that the production of 

 chlorophyl is prevented." And another 

 important fact has been proved — namely, 

 that plants bend themselves as much 

 from red light as they bend towards 

 white light in ordinary houses. To this 

 another interesting circumstance may be 

 added, that plants in a flowering state 

 continue much longer in bloom under 



