?8 THE GARDENER. [jAX. 



dung round your tender plants to impart warmth to 

 them, and some awning across hoops over your ranun- 

 culus heds, &c. and any of the bulbs above ground or 

 commg up, or else lay straw over them, while there 

 is any danger from frost. It may not be amiss to 

 mention also, that it is better to cover the plants which 

 you want to protect loosely than closely — compression 

 is bad, because the conducting power of bodies (as to 

 heat) is according to their thickness. The greatest 

 conductors of heat are metals, and the least so spongy 

 and light filamentous bodies, such as silk, cotton, 

 wool, &c. ; and it is on this account that such materials 

 constitute warm clothing — they give us a sensation of 

 warmth not by communicating heat to the skin, but by 

 preventing its escape into the air in consequence of 

 their non-conducting properties, when the air is expelled 

 by compression their conducting power is increased;* 

 therefore, as Mr. Loudon has inferred, '* in covering 

 plants or plant structures with leaves, litter, straw mats, 

 or other light porous bodies, the less they are compress- 

 ed the more effectual will they be found in preventing 

 the escape of heat. All tight coverings, whether of ani- 

 mals or plants, retain very little heat when compared 

 with loose coverings, and hence mats when drawn tightly 

 round bushes, or nailed closely against trees or walls, 

 are much less effective than when fastened over them 

 loosely, and do not retain nearly so much heat as a co- 

 vering of straw ; coverings of sand, ashes, or rotten 

 tan, applied to the ground, or to the roots of herba- 

 ceous plants, are for the same reason much less effec- 

 tive than coverings of leaves so applied, and these 

 again are much less so than coverings of litter or long 

 straw.'' It is necessary to observe, that rotten tan 

 should not be generally employed for the purpose of 



♦ See the article Heat, in the Library for the Diffusion of 

 Useful Knowledge* 



