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permeation of rays of their own colour. Thirdly, to inquire into the conditions required for the Palm House at Kew.
1. The result of my experiments, continued over some years, is to prove that the chemical rays of the solar spectrum, which are represented in the greatest power by the blue, indigo and violet rays, are the most active in producing the germination of seeds, and assisting the young plant up to the perfect development of its first leaves. After this period these rays become too exciting, producing the same effect on plants as pure oxygen gas would on animals. They induce rapid growth, and the leaves of the plant assume a healthful colour; but the perfect development of its parts is prevented, and the flowering and fruitbearing processes are checked, in many cases destroyed, the plant perishing rapidly after a certain period of its growth. Under the isolated influence of the luminous rays, represented by the orange and yellow rays, seeds will not germinate. But, after germination has taken place, and the plumule formed under the chemical or blue rays, I have found that the luminous or yellow rays are not injurious.
Indeed, experiments appear to prove they are more adapted for producing woody fibre than any other class of rays. The following results, the average of many experiments, will show this— Plants growing under the influence of blue rays left of carbon 7.16 per cent.
- 7.25 Ditto « »
green rays - - 760 » Ditto
- 7.69 » The effect of the red or calorific rays is to produce rapid evaporation from the soil and the surface of the plants ; even when this evaporation is met by an increased supply of moisture, germination is much retarded, and the young plant grows slowly, its leaves assuming a brown or red tint, showing that the chlorophyl—the colouring matter of healthy leaves -is prevented from forming.
The green rays may be regarded as preserving a position between the luminous and the chemical rays; and under certain conditions, into which it is not now necessary to enter, they produce very favourable results upon plants. It will be seen that these peculiar actions of the dissevered rays bear directly upon the conditions of plants during the spring, summer, and autumnal seasons ; and the conditions of the solar radiations during these periods afford satisfactory evidence in support of the correctness of the experiments named. In the spring, the luminous and thermic power of the sun-rays is less than their chemical influence. Germination and all the processes of a new growth are excited. In the suinmer, light and heat, increasing, act as interfering agents; the chemical excitation is subdued, and the processes of the formation of woody fibre are in full activity. The ripening of grain and fruits, depending more upon the calorific rays, ensues with the autumn, when the luminous effects and certainly the chemical powers of the solar forces are lessened.
2. It being impossible to keep the isolated prismatic rays for any prolonged period, or in sufficient quantity for satisfactory experiment in action upon plants, recourse was had to various transparent media-glass and coloured Auids held in glass cells. In most cases, I gave the preference to coloured fluids in my own experiments, as they allowed of my
adjusting their tints to any point of the chromatic scale. At the same time I used coloured glasses extensively, and with care I have been enabled to procure well-insulated rays in tolerable purity.
But it must be distinctly understood that the colour of any medium is not to be regarded as representing the colour of the rays by which it is permeated.
Cobalt-blue glass admits, besides the blue class of rays, the passage of the green, and a large portion of the red rays. Deep-red glasses may be permeated by yellow and blue rays, and yellow glasses are freely passed by blue and red rays, beyond the orange and yellow class. It will, therefore, be necessary that any kind of glass selected for a particular purpose should be examined as to the relation it bears to the coloured rays of the solar spectrum. If this is not attended to, the result may be very different from that anticipated.
3. It is required, according to Sir W. Hooker's letter of the 23rd of November, that a glass should be selected for the Palm House at Kew, “ so much stained as will deprive the glass of its scorching character and not affect the vegetation.” Two conditions are here required to be met. One, the production of a glass which shall obstruct the more intense thermic rays, and the other a glass which shall not affect the vegetation.
One only of these conditions is complied with in the use of glasses of a“ faint purple," as suggested by Sir W. Hooker, or of the “ violet tinge or azure hue" recommended by Mr. Turner. Any of these colours would, by obstructing light, give an increased action to the chemical principle of the sun-beam, and consequently excite an unnatural growth in the palms.
If a green glass of the proper tint is selected, all the conditions are complied with. The experiments of M. Melloni first proved the influence of green colour in obstructing the free permeation of radiant heat through transparent bodies. My own experiments, in particular reference to the growth of plants, have in the fullest degree confirmed this.
Green glass coloured with oxide of copper to a tint which may be called a pea-green will admit light and chemical power in the same proportions as white glass, but it will obstruct the passage of those rays which produce the “ scorching” desired to be avoided. I, therefore, should recommend for the Palm House, considering the probable conditions of the solar radiations over the countries-Italy, Sicily, and Barbary, to which the palm is indigenous—the use of a medium for glazing it, which would not interfere with the light or chemical power of the sun's rays, but which would obstruct a particular class of heating rays; namely, a tested green glass.
For the germination of seeds, the striking of cuttings, &c., a green or a hot-house glazed with a pure blue glass would be desirable, and also for the cultivation of alpine plants, which are more exposed in nature to the action of the unabsorbed chemical radiations from an intense blue sky. Our object in all cases appears to be to endeavour to imitate, not to force the conditions of nature.
ROBERT Hunt. Sir Henry De la Beche. * I have been led to doubt the correctness of this part of my recommendation since that time.
Museum of Economic Geology,
6th December, 1845. MY LORD,-Having referred the consideration of the kind of coloured glass desirable to be employed in the Kew Palm House, to Mr. Hunt, in conformity with your Lordship’s directions, I have the honour herewith to transmit his report on the subject.
[Signed] H. T. De la BECHE. The Rt. Hon. The Earl of Lincoln.
Office of Woods, &c.,
9 July, 1846. Sir, — Adverting to the report which you made in December of last year, in reply to the reference as to the kind of coloured glass desirable to be employed in glazing the New Palm House in the Royal Botanic Gardens at Kew, I am, in behalf of the Commissioners of Her Majesty's Woods, &c., to acquaint you, that as the time has now arrived for deciding as to the description of glass to be adopted, the Board, agreeably to your recommendation, desire to adopt a glass coloured with oxide of copper, to the tint commonly called pea-green; but as the adjustment of the tint may be of great importance, and involve some nicety in matters of which they consider you are particularly competent to judge, the Board have directed Mr. Decimus Burton, the architect of the building, and Mr. Turner, the contractor for the works, to place themselves in communication with you, so that under your instructions a specimen may be procured to your satisfaction; and I have therefore to request the favour of your assistance in the course of proceeding the Board purposes to adopt.
I am, sir, &c.,
· [Signed] A. MILNE. Robert Hunt, Esq.
Museum of Economic Geology,
December 16. 1846. My LORD AND GENTLEMEN, — Agreeably to the request of Mr. Milne, as conveyed in his letter of the 9th July, I placed myself in communication with Mr. Decimus Burton, and Mr. Turner, on the subject of the tinted glass, which the Board, on the recommendation of my memorandum of the 6th December, 1845, appears desirous of adopting in the new Palm House at Kew.
I have been supplied by these gentlemen with thirty-seven specimens of green glass, the whole of which I have submitted to an experimental examination, the purpose of which was to ascertain the relative amount of obstruction which they offered to the passage of the solar-heat rays, which Sir William Hooker, in his letter of the 23rd November, 1845, says “scorches the foliage of the plants.”
I found in these examinations, which consisted of measuring off the various thicknesses of the coloured bands of the prismatic spectrum, and of experiments on their chemical and thermic action, after they had permeated these variously tinted green media, that it was quite within my power to cut
off nearly the whole of the sun's heating rays, without obstructing much of the light. Experiments in which I have been engaged for some years, prove that one class of the sun's rays materially quicken vegetation ; and that under their influence plants might be excited into unnatural growth; whilst to another class a restraining power belongs, by which this stimulating principle is modified, and under their united influences the healthful growth of the plant is secured. It therefore became important to discover a medium which should not disturb the natural proportions of these classes of rays, the unequal permeation of which would be injurious to the palms, but which should at the same time prevent that intense action of the heat rays, which is already complained of, and which in the new Palm House would, in all probability, be much more energetic, owing to its form. Another point which I felt it necessary to regard with particular attention, was the general appearance of so large and so ornamental a structure as that which is now being built in the Royal Botanic Garden. I have the satisfaction of being enabled to report that I have selected a glass which, from its peculiar composition, secures the required condition for the plants, and which is so slightly tinted, as to present no appearance by either reflected or transmitted light materially different from the white sheet glass already employed for glazing the stove house for the orchideous plants at Kew; and which is more trans. parent than much of the crown glass in the old houses in these gardens.
This glass admits most freely the permeation of all the luminous and chemical rays, and obstructs only those heat rays which are remarkable for their peculiar scorching power. The natural conditions of the plant are not at all interfered with; and it is protected from those circumstances arising from the refraction and concentration of beat rays by the ordinary glass, which place the foliage of the plant in an unnatural position. With this, I beg to submit to the Board a piece of the glass which I have selected. In the event of its being adopted, it would be necessary to require from the manufacturer, that no oxide of manganese should enter into the composition of the glass ; the effect of which would be a gradual whitening under the agency of light, which would destroy the property for which it is now chosen.
May I be permitted to remark, that my attention has been directed to a destructive influence exerted along two peculiar, and as it appears wellmarked lines in the house for the orchideous plants. This, from an examination I have made, appears to arise from the actual formation of a prism of vapour along the angular roof, and I have no doubt but the lines alluded to mark out the true angles of refraction for the heating rays thus prismatically divided from the others. In all probability a roofing of green glass to this house would prevent this effect, and secure the shade which most of the orchideous plants seek for in nature. The experiment might be tried at a very trifling expense, and the result would be instructive.
I am, my lord and gentlemen,
The Commissioners of Woods, 8c.
Office of Woods, &c.,
21st Dec. 1846. Sir, I have, on behalf of the Commissioners of Her Majesty's Woods, &c., to acknowledge the receipt of your letter of the 16th instant, together with a specimen of the tinted green glass which you recommend to be adopted for glazing the New Palm House in the Royal Botanic Gardens at Kew; and as the Board are extremely desirous that this matter should be well considered, they have referred your report and specimen of glass to Sir William Hooker, for his advice, with reference to the effect upon the plants, and to Mr. Burton, with reference to the appearance in connection with the building. The Board will therefore feel obliged if you will give these gentlemen the benefit of your opinion on this subject, and conjointly with them submit a report to the Board.
I am, &c.,
[Signed] Charles GORE. Robert Hunt, Esq.
Memorandum of the principles upon which the glass has been selected for
the Palm House in the Royal Botanic Gardens at Kew. The object to which my attention was particularly directed, was the selection of a glass which should not be in any way injurious to the vegetation of the plants, but which should at the same time protect them from the heat rays, which are found to permeate pure white glass, and speedily destroy the colouring matter of the leaves.
The tinted green glass, on the beneficial action of which I have the greatest reliance, having been considered by Sir Wm. Hooker, as from his experience likely to prove most advantageous in the Palm House, and being regarded as unobjectionable by Mr. Decimus Burton, it has been thought advisable by these gentlemen that the Board should be placed in possession of the principles and experiments which led me to select the glass in question.
The scorching of the leaves—the evil complained of- is due entirely to the influence of a class of Heat rays, which are distinguished by their red colour, or which exist in that portion of the prismatic spectrum where there are no Light rays.
This I have proved to be the case by experiments which have been extended over many years, and it has also been discovered by Sir John Herschel, that all vegetable colours are destroyed by that ray the colour of which is complementary to that of the leaf or flower—hence the green colouring matter of leaves is destroyed by the red rays.
It became necessary therefore to select a glass which should not admit the permeation of the more intense heat rays, allowing at the same time