142 ON THE METHOD OF WARMING STOVES. 



being no such thing as a natural hot-bed. This appears to me to have 

 been asserted without duly considering that plants in a hot-house are 

 in a situation altogether different from what they would be out of doors 

 in their native climate, particularly with regard to the state of the 

 atmosphere in which they grow. 



Air is an elastic fluid which expands by heat, therefore all particles 

 of it, as they become warm, unless they meet with some external 

 impediment, will ascend till they reach a stratum of similar density to 

 themselves ; the heat will consequently always be greatest at the 

 radiating or reflecting surface : hence the earth at any given place, 

 unless cooled by evaporation or some accidental cause, will be warmer 

 than the air immediately above it, and this again will be warmer 

 than portions of the atmosphere more remote : this is very sensibly 

 felt in places at any considerable variation of altitude. Now, although, 

 for all horticultural puposes, owing to the comparatively small height 

 of any vegetable production, the temperature at the same time and 

 place may be considered as uniform, still the lower parts of the plants 

 are, if anything, rather in the warmer medium. Moreover in tropical 

 climates, the earth, from the great power of the sun's rays, and their 

 continued action, becomes heated to a considerable depth. Now in all 

 horticultural stoves the heat will be found to vary by a law exactly 

 the reverse of this which obtains in nature. Here the heated particles, 

 being intercepted in their ascent, and confined by the glass roof, the 

 top of the house, as practical men know well to be the case, will 

 always be warmest, and the temperature will rapidly decrease towards 

 the bottom, and nearly in a ratio proportionate to the degree of heat 

 maintained ; hence the necessity for a permanent source of heat at 

 the bottom, not to keep the root warmer than the rest of the plant, 

 but merely to obviate its being in a colder situation. A mild bottom 

 heat accordingly is always found in practice to succeed best. For the 

 same reason, unless the plants are kept very near glass, a great cir- 

 culation of fresh air, and consequent waste of heat, is generally found 

 necessary, as, unless the heated air at the top was thus suffered to 

 escape, the leaves and extremities of the plants, being attracted by 

 the warmer medium above them, would grow towards it faster than 

 the lower parts could supply nourishment, and thus would become 

 what gardeners term drawn. The necessity for change of air, except 

 in reference to temperature and moisture, cannot well be accounted 



