When a temperature of 45°, moisture, and atmospheric air occur to deaden vegetable matters, these absorb large quantities of oxygen, evolving also an equal volume of carbonic acid. As in all other instances where vegetable substances absorb oxygen gas in large quantities, much heat is evolved by them when putrefying; and advantage is taken of this by employing leaves, stable-litter, and tan, as sources of heat, or hot-beds, in the gardener's forcing department.

A hot-bed is usually made of stable-dung, of which that made by the best fed horses is to be preferred. It should be about ten days from the stalls, and without too large a proportion of litter. After being thrown into a heap, of conic form, for five or six days, it must be so turned over, that the inner parts are brought, to the outside, the clots well separated with the fork, the heap being re-formed conically as before, and left for an equal number of days. By this time and treatment the dung in general acquires a sufficient and steady heat; if, however, it is very dry and fresh, it must be moderately moistened, and left for five or six days more. At the time of forming the heap, as well as at every turning, water should be applied if its substance appears at all dry, as a regular state of moisture is of first importance to the obtaining a favourable fermentation. It should remain until the straw in general assumes a dark brown colour, when it should be immediately formed into the bed. Leaves or tan may be mixed with advantage, as heat is thereby generated during a greater length of time.

In cold, wet, or boisterous weather, the heaps should be covered to a moderate depth with litter.

In making the beds, they must be so situated as to be entirely free from the overshadowing of trees, buildings, etc, and having an aspect rather a point eastward of the south. A reed fence surrounding them on all sides is a shelter that prevents any reverberation of the wind, an evil which is caused by paling or other solid inclosure. This must be ten feet high to the northward or back part, of a similar height at the side, but in front only six. The wicket or gate must be of sufficient width to admit a loaded wheelbarrow. An inclosure of this description, one hundred feet in length and sixty broad, will be of a size sufficiently large for the pursuit of every description of hot-bed forcing. But for cucumbers, melons, and a few inferior articles, a space for six or eight lights is sufficient. Fruit may be forced slightly by being trained within it on the southern aspect; the fence on that side in that case must be of brick or wood.

To prevent unnecessary labour, this inclosure should be formed as near to the stable as possible. For the recep-tion of the bed, a trench is often dug of its determined length and breadth, and six inches deep, if the soil is wet, or eighteen or more if it is dry. In a dry soil and climate this cannot be pro-ductive of much injury, but otherwise it almost always chills the bed: at the same time it is to be observed, that it is never productive of benefit, further than not being so high; it is easier of access, but gives much additional trouble, both at the time of founding and afterwards, when linings are to be ap-plied.

The site of the bed being determined, a stake should be driven perpendicu-larly at the four corners as a guide for its rectangular construction. The dung must be thoroughly mixed just before it is used, and as carefully separated and spread regularly with the fork, as the bed is formed with it. It is beneficially settled down in every part alike by beating with the fork as the work pro- ceeds, rather than by treading; for if too much compressed, a high degree of heat is generated but is soon spent: a contrary phenomenon is often caused if trod to a still greater excess, namely, that no heat at all is engendered.

The longest or littery part of the dung should be laid at the bottom of the bed, and the finer fragments of the dung upon the top. If it is not regularly and moderately moist throughout, it should be sprinkled over with water. As the surface on which the bed is founded is usually horizontal, so is the dung laid perfectly parallel with it. Mr. Knight recommends it, on the contrary, to be equally inclined with its foundation, that it may associate well with the new form, which he recommends for frames. See Frame.

The breadth of a bed must always be five feet, and in the depth of winter four and a half feet high when firmly settled; to form it of this size, about twelve barrow loads of dung are required to a light.

In early spring, a height of three and a half feet is sufficient, and as the season advances, it may decline to three or two and a half feet. In May or early summer, when the only object is to hasten the germination of seeds, two feet or eighteen inches is not less than the necessary height. The length of 20 the bed in all cases must be guided by the size of the frame.

To prevent the sudden changes of temperature in the external air affecting the heat of the bed, coat the sides of the bed with sand; coal-ashes or earth might be substituted, to a thickness of two feet.

As the heat declines, linings, or as they might be more properly called, coatings, are made use of, which consist of hot fermenting dung laid from eighteen to twenty-four inches, in proportion to the coldness of the season, etc, all round the bed to the whole of its height, and if founded in a trench, one equally deep must be dug for the coating, it being of importance to renew the heat as much as possible throughout its whole mass; if, after a while, the temperature again declines, the old coating must be taken away, and a similar one of hot dung applied in its place. As the spring advances, the warmth of the sun will compensate for the decline of that of the bed; but as the nights are generally yet cold, cither a moderate coating, about nine or ten inches thick, is required, or the mowings of grass, or even litter, may be laid round the sides with advantage.