Note 44 — Chapter VI

“The physical characters of this species of glass, which acts so differently from the other species of coloured glass in all the phenomena of calorific absorption, are, 1st, its intercepting almost totally the rays which pass through alum; 2nd, its entirely absorbing the red rays of the solar spectrum. I have already stated that their colouration is produced almost entirely by the oxide of copper.

“Thus, the colouring matters of the coloured glasses, while they so powerfully affect the relations of quantity which the different rays of ordinary light bear to each other, exercise no elective action on the concomitant calorific rays. This curious phenomenon is the more remarkable as the colouring matters absorb almost always a very considerable portion of the heat naturally transmitted by the glass. The following are, in fact, the calorific transmissions of the seven coloured glasses referred to; the transmission of the common glass being represented by 100; red glass, 82·5; orange, 72·5; yellow, 55; bluish-green, 57·5; blue, 52·5; indigo, 30; violet, 85. The quantity of heat absorbed through the action of the colouring substances is, therefore, 17·5 in the red glass, 27·5 in the orange, 45 in the yellow, 42·5 in the green, 47·5 in the blue, 70 in the indigo, and 15 in the violet. Now, as these absorptions extinguish a proportional part of each of the rays which constitute the calorific stream transmitted by common glass, they may be compared, as we said before, with the absorbent action exercised on light by matters more or less deeply brown or dark, when they are immersed in water, or some other colourless liquid which dissolves, but does not affect them chemically.”—Annales de Chimie et de Physique, tom. xl. p. 382.

Guided by these principles, the author selected the glass employed in glazing the Royal Palm-House, at Kew Botanical Gardens, where it was desired to obstruct the passage of those rays which have a particular scorching influence. Of this glass a description was given at the meeting of the British Association at Oxford, which appears in the Transactions for that year. The result has been all that could be desired—not a single instance of scorching having occurred during the three years which have elapsed.