392 ACCELERATING VEGETATION. 



of trees, and especially oak -leaves, mown grass, weeds, clippings of hedges, 

 and almost every other article capable of putrescent fermentation, may be 

 used either alone or with stable-dung. Tanners' bark, or oak-leaves, are 

 found the preferable fermenting materials for hot-beds in hot-houses, because 

 they undergo less change in bulk, and retain their heat longer than dung 

 or any other fermentable substance that can be readily obtained in equal 

 quantities. Leaves do not produce such a powerful heat as bark, but they 

 have this advantage, that when perfectly decayed, they form a rich mould, 

 which is useful both as soil and as manure ; while rotten tanners' bark is 

 found rather injurious than useful to vegetation, unless it be well mixed 

 with lime or with earth, or left till it is thoroughly decayed into mould. 

 When it ceases, therefore, to be used in the hot-house or hot-bed, it is 

 employed in the open garden as a surface-mulching, to keep in heat or 

 moisture. 



842. Preparation of materials for hotbeds. The object being to get rid of 

 the violent heat which is produced when the fermentation is most powerful, it 

 is obvious that preparation, whether of leaves, tan, and stable dung,must consist 

 in facilitating the process. For this purpose, a certain degree of moisture 

 and air in the fermenting bodies are requisite ; and hence the business of the 

 gardener is to turn them over frequently, and apply water when the process 

 appears impeded for want of it, and exclude rain when it seems chilled and 

 retarded by too much water. Recent stable-dung generally requires to lie a 

 month in ridges or beds, and be turned over in that time thrice before it is 

 fit for cucumber beds of the common construction ; but for M'PhaiTs hot- 

 beds, or for linings or casings, or any description of hot-bed or pit, no time 

 at all need in general be given, but the dung formed at once into linings. 

 Tan and leaves require in general a month to bring them to a proper degree 

 of heat ; but much depends on the state of the weather and the season of the 

 year. Fermentation is always most rapid in summer ; and if the materials 

 are spread abroad during frost, it is totally impeded. In winter, the process 

 of preparation generally goes on, under cover from the weather, in the back 

 sheds ; which situation is also the best in summer, as full exposure to the 

 sun and wind dries too much the exterior surface ; but where sheds cannot 

 be had, it will go on very well in the open air. A great deal of heat is un- 

 doubtedly lost in the process of fermentation ; and some cultivators have 

 recently devised plans to turn it to some account, by fermenting dung in 

 vineries which are just beginning to be forced, or in vaults under pine -pits 

 or plant stoves. The latter mode seems one of the best in point of economy,^ 

 and is capable of being turned to considerable advantage, where common 

 dung-beds are extensively used ; but the most economical plan of any is un- 

 doubtedly that of employing M'Phail's pits, or such as are constructed on 

 similar principles. 



843. M l Phail's hotbed or pit consists of two parts, the frame and lights of 

 which are of wood, and not different from those used for growing cucumbers, 

 or other ordinary purposes, and the basement on which the frame is placed, 

 which is flues of brickwork, with the outer wall uniformly perforated, or as 

 it is commonly called pigeon-holed, as shown in fig. 135, in p. 196. Against 

 these perforated flues linings of dung are formed, the steam of which enters 

 the flue, and heats the earth inclosed. The chief objections to this plan are 

 the first cost and the greater consumption of dung, which some allege is 

 required to keep up the proper heat. Its advantages are, that hot dung may 



