252 ON THE COLOURED GLASS EMPLOYED IN GLAZING, &C. 



ON THE COLOURED GLASS EMPLOYED IN GLAZING \ 

 THE NEW PALM HOUSE IN THE ROYAL BOTANIC 

 GARDENS AT KEW. 



BY B. HUNT. 



It has been found that plants growing in stove houses often suffer 

 from the scorching influence of the solar rays, 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 Avhich 

 should be effective, this building being 363 feet in length, 100 feet 

 wide, and 63 feet high. It was therefore tiiought desirable to ascer- 

 tain if it would be possible to cut off these scorching rays by the use 

 of a tinted glass, whicli should not be objectionable in its appearance ; 

 and the question was, at the recommendation of Sir Wm. Hooker and 

 Dr. Lindley, submitted by the Commissioners of Woods, &c., to 

 Mr. Hunt. The object was to select a glass which should not permit 

 those heat rays which are the most active in scorching the leaves of 

 plants to permeate it. By a series of experiments made with the 

 coloured juices of the Palms themselves, it was ascertained that the 

 rays which destroyed their colour 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 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 Mere examined obstructed nearly all the heat rays; 

 but this was not desired, and from their dark colour these were objec- i 

 tionable, as stopping tlie passage of a considerable quantity of light, 

 Avhich was essential to the healtliful 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 tlie desideratum has 

 been arrived at. Every sample of glass was submitted to tiiree distinct 

 sets of experiments. 1st. To ascertain, by measuring off tlie coloured 

 rays of the spectrum, its transparency to luminous influence. 2nd. To 

 ascertain the amount of obstruction offered to the passage of the 

 chemical rays. 3rd. To measure the amount of heat radiation which 

 permeated each specimen. The chemical changes were tried upon 

 chloride of silver, and on papers stained with the green colouring 

 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 

 smoked on one side by holding it over a smoky flame, and then w'hile 

 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 oft' in well 

 defined circles long before the other parts presented any appearance of 

 dryness. By these means it was not diflRcult, with care, to ascertain 

 exactly the conditions of the glass, as to its transparency to light, heat, 

 and chemical agency (actinism). The glass thus chosen is of a very 



