Scientific American 21, 12.2.1848
At the last meeting of the British Association for the advancement of Science, R. Hunt, Esq presented the following interesting statistics in relation to various experiments with colored glass: —
"On the colored glass employed in glazing the new Palm house in the Royal Botanic Garden at Kew. It has been found that plants grooving in stove houses often suffer from the scorching rays of the solar says, and great expense is frequently incurred in fixing blinds to cut off this destructive calorific influence. From the enormous size of the new Palm house at Kew, it would be almost impracticable to adopt any system of shades which would be effective — this building being 364 feet in length, 100 feet wide, and 63 feet high. It was, therefore, thought desirable to ascertain if it would be possible to cut off these scorching rays by the use of a tinted glass, which should not be objectionable in its appearance, and the question was submitted to Mr. Hunt. The object was to select a glass that should not permit those heat rays which are most active in scorching the leaves of plants to permeate it. By a series of experiments made with the colored juices of the palms themselves, it was ascertained that the rays which destroyed their color, belonged to a class situated at that end of the prismatic spectrum which exhibited the utmost calorific power, and just beyond the limits of the visible red ray. A great number of specimens of glass variously manufactured were submitted to examination, and it was at length ascertained that glass tinted green appeared likely to effect the object desired most readily. Some of the green glasses which were examined obstructed nearly all the heat rays — but this was not desired — and from their dark color these were objectionable, as stopping the passage of a considerable quantity of light, which was essential to the healthful growth of the plants. Many specimens were manufactured purposely for the experiments by Messrs. Chance of Birmingham, according to given directions, and it is mainly due to the interest taken by these gentlemen that the desideratum has been arrived at. Every sample of glass was submitted to three distinct set of experiments — 1st. To ascertain, by measuring off the colored rays of the spectrum, its transparency to luminous influence. 2. To ascertain the amount of obstruction offered to the passage of the chemical rays. 3. To measure the amount of heat radiation which permeated each specimen The chemical changes were tried upon chloride of silver, and on paper stained with the green coloring matter of the leaves of the palms themselves.
The calorific influence was ascertained by a method employed by Sir John Herschel in his experiments on solar radiation. Tissue paper stretched on a frame was smokes' on one side by holding it over a smoky flame and then while the spectrum was thrown upon it the other surface was washed with strong sulphuric ether. By the evaporation of the ether the points of calorific action were most easily obtained, as these dried off in welldefined circles long before the other parts presented any app earance of dryness. By these means it was not difficult, with care, to ascertain exactly the condition of the glass, as to its transparancy to light, beat, and chemical agency. The glass thus chosen is of a very pale yellow-green color, the color being given by oxide of copper, and is so transparent that scarcely any light is intercepted. In examining the spectral rays through it, it is found that the yellow is slightly diminished in intensity, and that the extent of the red ray is affected in a small degree, the lower edge of the red ray being cut off by it. It does not appear to act in any way upon the chemical principle, as spectral impressions obtained upon chloride of silver are the same in extent and character as those procured by the action of the rays which have passed ordinary white glass. The glass has, however, a very remarkable action upon the nonluminous heat rays, the least refrangible calorific rays. It prevents the permeation of all that class of heat rays which exists below and in the point fixed by Sir William Herschell, Sir H. Englefield and Sir J Herschell, as the point of maximum caloric action. As it is to this class of rays that the scorching influence is due, there is every reason to conclude that the use of this glass will be effective in protecting the plants, and at the same time, as it is unobjectionable in point of color, and transparent to that principle which is necessary to the developement of those parts of the plant which depend upon external chemical excitation, it is only partially so to the heat rays, and it is opaque to those only which are most injurious. The absence of the oxide of manganese, commonly employed in all sheet glass is insisted on, it having been found that into glass, which manganese enters the composition of, 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 this glass is chosen. Melloni, in his investigations on radiant heat, discovered that a peculiar green glass, manufactured in Italy, obstruvted nearly all the calorific rays; we may therefore conclude that the glass chosen is of a similar character to that employed by the Italian philosopher. The tint of color is not very different from that of the old crown glass, and many practical men state that they find their plants flourish much better under this kind of glass, than under the white sheet glass which is now so commonly employed.
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