HEAT, CONSIDERED WITH REFERENCE TO HORTICULTURE. 



67 



CHAPTER IV. 



THE ATMOSPHERE, CONSIDERED WITH REFERENCE TO 

 HORTICULTURE. 



217. The influeDce of the atmosphere on the geographical distribution 

 of plants has been noticed in a preceding chapter (147), and we shall here 

 consider the subject with reference to the culture of plants in gardens, 

 taking as our guide, Daniel's Essay on Climate with regard to Horticulture, 

 ( Hort. Trans, vol. vii., J Darnell's Meteorological Essays^ and examining 

 also what has been -written on the subject in subsequent works. Among 

 the latter may be mentioned Howard's Climate of London^ Hutchison's 

 Treatise on Meteorological Phenomena, Murphy's Meteorology, and two 

 excellent articles on the two latter of these works in the Athenceum for 1837, 

 p. 561 and 580. 



The atmosphere on every part of the globe consists of the same consti- 

 tuent parts, to wit, carbonic acid gas and water in a state of vapour about 1 

 part, oxygen 23, and azote or nitrogen 76, reckoning by weight. The 

 aqueous vapour and carbonic acid gas are variable admixtures ; but in all 

 cases they bear only a very small proportion to the other ingredients. All 

 the variations, tlieretbre, which are found in the atmosphere in different 

 countries, and at different times in the same country, depend upon the 

 modifications impressed upon it by heat, moisture, motion, and light. 



Sect. L — Heat, considered with reference to Horticulture. 



218. Heat, like light, is found to be capable of radiation, reflection, 

 transmission 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 transmitted 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 exemplified in the case of the common fire-screen. The diff'usion of 

 heat by conduction and radiation is what chiefly concerns the horticulturist. 



219. 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 thermo- 

 meter placed on the metal will rise soonest ; next, that placed on the stone; 

 next, that on the glass ; then that on the wood ; and lastly, that on the 

 earth. The conducting power of bodies is generally as their density. The 

 greatest of all conductors of heat are metals ; and the least so, spongy and 

 light filamentous bodies. Silk, cotton, wool, hare's fur, and eider-do\\Ti, 

 are extremely bad conductors of heat, and hence their value as clothing. 

 {Library of Useful Knowledge, art. Heat, p. 23.) They give us a sensation 



f2 



