GLASS, COLOURED 673 



exactly adjusted to produce the best possible conditions ; but under some of the artificial 

 conditions in which we place plants, it is important to know the conditions of the 

 solar rays best suited to produce a given effect. This we must attempt briefly to 

 explain : 



1. Seeds germinating absorb oxygen, and convert their starch into sugar; this is a 

 purely chemical process, and demands the full power of the chemical rays (actinism). 



2. Wood-forming, from the decomposition of carbonic acid, is a function of tho 

 vital power of the plant, excited by light (luminous force). 



3. Flowering and fruiting manifest compound actions, and appear to demand the 

 combined power of heat (calorific power) and of the chemical rays. 



Such are the three chief conditions in the phenomena of vegetable growth. Now 

 a, a glass stained blue with cobalt admits the permeation of the chemical rays with 

 great freedom, obstructing both light and heat; b, a glass stained yellow with 

 silver, will powerfully obstruct the chemical rays, and allow the luminous rays to 

 pass freely; c, deep copper or gold red glasses admit the maximum heat rays to 

 pass freely, and in general allow of the permeation of a small quantity of the chemical 

 rays. 



When seed is placed in the soil to germinate, a blue glass placed above the soil 

 will greatly accelerate the process; the first leaves will appear above the soil, in 

 many instances, days before they are seen when the seed is under the ordinary 

 conditions in the soil ; but if a plant is allowed to grow under these circumstances, 

 scarcely any wood is produced, but long succulent stalks are formed, with imperfect 

 leaves. 



After germination has taken place, if the plant is brought under the influence of 

 the rays permeating yellow glass (light separated to a considerable extent from the 

 chemical power), wood is formed abundantly, and very healthy plants with dark leaves 

 are produced. For the production of perfect flowers and fruit, the red glass named 

 is the most effective. Plants growing in conservatories which have been glazed with 

 the colourless German sheet-glass, frequently suffer from scorching. To avoid this if 

 possible, the editor of this volume was consulted on the glass which should be 

 employed in glazing the great palm-house at Kew, the problem being to avoid the 

 necessity of blinds, and to secure the plants from the injurious action of the scorching 

 rays. By a long series of experiments it was determined that glass stained with a 

 little of the oxide of copper, and from which there was an entire absence of man- 

 ganese, entirely effected this end. The great palm-house in the Koyal Botanic 

 Gardens at Kew was glazed with glass made on this principle, by the Messrs. Chance 

 Brothers and Co. of Birmingham, and it has now been tested by the sunshine of 

 six-and-twenty summers (1874); and the plants, as every one may observe, grow 

 most luxuriantly, and are entirely free from any indications of scorching on their 

 leaves. 



GLASS, COLOURED. Most of the metallic oxides impart a colour to glass, 

 and some non-metallic, and even some substances derived from the organic kingdom 

 have the power of imparting permanent colours to the vitreous combinations of 

 flint and potash. There is much in this subject which still requires examination. 

 M. Bontemps, at the meeting of the British Association at Birmingham, brought 

 forward some very extraordinary facts in connection with the colouring powers of 

 different bodies. Of his communication the following is an abstract : 



In the first place it was shown, that all the colours of the prismatic spectrum 

 might be given to glass fey the use of the oxide of iron in varying proportions, and 

 by the agency of different degrees of heat : the conclusion of the author being, that 

 all the colours are produced in their natural disposition in proportion as you increase 

 the temperature. Similar phenomena were observed with the oxide of manganese. 

 Manganese is employed to give a pink or purple tint to glass, and also to neutralise 

 the slight green given by iron and carbon to glass in its manufacture. If the glass 

 coloured by manganese remains too long in the melting-pot or the annealing-kiln, 

 the purple tint turns first to a light brownish-red, then to yellow, and afterwards to 

 green. White glass, in which a small proportion of manganese has been used, is liable 

 to_ become light yellow by exposure to luminous power. This oxide is also, in certain 

 window-glass, disposed to turn pink or purple under the action of the sun's rays. 

 M. Bontemps has found that similar changes take place in the annealing oven. He 

 has determined, by experiments made by him on polygonal lenses for M. Fresnel, 

 that light is the agent producing the change mentioned : and the author expresses a 

 doubt whether any change in the oxidation of the metal will explain the photogenic 

 effect. A series of chromatic changes of a similar character were observed with the 

 oxides of copper, the colours being in like manner regulated by the heat to which 

 the ^ glass was exposed. It was found that silver, although with less intensity, ex- 

 hibited the same phenomena ; and gold, although usually employed for the purpose 



VOL. II. XX 



