Mr. Knight adds, “I have not yet been troubled with insects upon my Pine plants (having only had nine plants for about as many months), and have not, of course, tried any of the published receipts for destroying them. Mr. Baldwin recommends the steam of hot fermenting horse-dung: I conclude the destructive agent, in this case, is ammoniacal gas; which Sir Humphry Davy informed me he had found to be instantly fatal to every species of insect; and if so, this might be obtained at a small expense, by pouring a solution of crude muriate of ammonia upon quick-lime; the stable, or cow-house, would afford an equally efficient, though less delicate, fluid. The ammoniacal gas might, I conceive, be impelled, by means of a pair of bellows, amongst the leaves of the infected plants, in sufficient quantity to destroy animal, without injuring vegetable life: and it is a very interesting question to the gardener, whether his hardy enemy, the red spider, will bear it with impunity.” [Illustration: 18] In the year 1820, in June, Mr. Knight had such a house as he has hinted at, erected. Its general appearance (fig. 18.), is simple, and the roof admits as much light, as any roof that can be constructed in the present state of knowledge, in the combination of wrought iron and common glass. [Illustration: 19] The plan of this house, or pit (fig. 19.), is fifty feet in length, and ten feet wide; the furnace (_a_) is placed at one end; the flue proceeds from it directly to the front parapet (_b_), and passing along close under it to the opposite end, there terminates in a chimney (_c_). Instead of a pit, a curious stage is constructed, by forming cross walls (_d_), or rather piers, connected by arches, and finished by a gradation of flat surfaces, or steps, on which the pots are placed, so as to stand as near the glass as possible (fig. 20.) [Illustration: 20] Air is admitted by shutters, which open outwards, immediately under the stone plinth of the parapet (fig. 20. _a_), in which the lower ends of the iron bars are fixed; and allowed to escape by similar shutters, opening outwards, immediately under the stone coping of the back wall (_b_), in which the upper ends of the same bars are leaded in. The path behind is on a level with the exterior surface; the width of the cross walls the length of a brick, or nine inches, and they are finished with foot tyles; the width between them is about fifteen inches, by which means, any ordinary sized person may pass from the back path to the front flue, and water or examine the plants on each side. This house being finished, was immediately stocked with Pines, some figs, and various other plants, all of which, Mr. Knight stated verbally, in May 1821, to various members of the Horticultural Society, succeeded admirably; but by neglect of the gardener, or rather labourer, who attended them, they were killed by an over-heat in Mr. Knight’s absence from home.[1]