26 HEAT, CONSIDERED WITH 



the same, but the percentage of aqueous vapour and carbonic acid 

 gas varies, and certain modifications take place in the air' as it is 

 affected by heat, moisture, motion, and perhaps light. 



Heat. 



Heat, like light, is found to be capable of radiation, reflection, trans- 

 mission through transparent media, and refraction ; but it is radiated, 

 reflected, transmitted, and refracted, in a different manner and degree 

 from light. Thus it appears that both light and heat can be trans- 

 mitted through either gaseous, fluid, or solid media, provided they are 

 transparent. Any opaque body is to light, however, an impenetrable 

 barrier ; but to heat, or to its conduction, neither opaqueness nor 

 solidity affords resistance. On the contrary, heat is conducted more 

 rapidly by solid than by fluid or gaseous bodies ; a fact which will be 

 noticed in treating of artificial coverings for protecting plants. A 

 solid body will obstruct the radiation of heat, as is familiarly exem- 

 plified in the case of the common fire-screen. The diffusion of heat by 

 conduction and radiation is what chiefly concerns the horticulturist. 



The conduction of heat is effected by the contact of bodies heated 

 in different degrees, when the tendency to equal diffusion immediately 

 raises the temperature of the one body and lowers that of the other. 

 This takes place with different degrees of rapidity, according to the 

 nature of the bodies in contact. If thermometers be placed on metal, 

 stone, glass, ivory, and earth, all heated from the same source, we 

 shall find that the thermometer placed on the metal will rise soonest ; 

 next, that placed on the stone ; next, that on the glass ; theu 

 that on the wood ; and lastly, that on the earth. The conducting 

 power of bodies is generally as their density. The greatest of all con- 

 ductors of heat are metals ; and the least so, spongy and light fila- 

 mentous bodies. Silk, cotton, wool, hare's fur, and eider-down, are 

 extremely bad conductors of heat, and hence their value as 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. The power which these bodies have of 

 stopping the transmission of heat depends on the air which is stag- 

 nated in their vacuities; for- when the air is expelled by compression, 

 their conducting power is increased. Hence, in covering plants or 

 plant structures with leaves, litter straw, mats, or other light, porous 

 bodies, the less they are compressed the more effective will they be 

 found in preventing the escape of heat by conduction. All tight cover- 

 ings, whether of animals or plants, retain very little heat, when com- 

 pared with loose coverings ; and hence mats, when drawn tightly 

 round bushes, or nailed closely against trees on walls, are much less 

 effective than when fastened over them loosely, and do not retain 

 nearly so much heat as a covering of straw. Coverings of sand, ashes, 

 or rotten tan, applied to the ground, or to the roots of herbaceous 

 plants, are, for the same reason, much less effective than coverings of 

 leaves so applied ; and these, again, are much less so than coverings of 



