REFERENCE TO HORTICULTURE. 27 



litter or long straw. The heat of the trunks of trees is prevented 

 from escaping to the extent it otherwise would do by their bark, 

 which is a powerful non-conductor, and the heat of the ground by a 

 covering of snow, which, by its spongy, porous nature, contains a 

 great deal of air. Without this covering, the herbaceous plants of 

 the northern regions could not exist ; nor would spring flowers, such 

 as the aconite, snow-drop, crocus, daffodil, &c., in the climate of Scot- 

 land, come nearly so early into bloom. 



Heat is diffused amongst bodies not in contact by the process called 

 radiation, in consequence of which property a person standing near 

 any body heated to a higher temperature than himself will experience 

 a sensation of warmth. The radiation of heat from any body proceeds 

 from its surface in every direction in straight lines, in the same manner 

 as the divergent rays of light from an illuminated body, as for example, 

 a lighted candle ; and rays of heat, like rays of light, may be reflected 

 from polished surfaces, and transmitted and refracted through trans- 

 parent substances, and even polarized. But though it be true that 

 heat, in proceeding from a body, begins by radiating from it at right 

 angles and in straight lines, yet this can only be strictly said of heat 

 which is radiated perpendicularly into the atmosphere. Thus, from a 

 pipe of water equally heated, the heat tends to radiate at right angles 

 from its surface in all directions ; yet none but those rays which pro- 

 ceed from the uppermost part of the convex surface of the pipe will 

 preserve their perpendicularity. All the other rays, from their first 

 contact with the air, will be deflected upwards, being in fact carried 

 in that direction by the heating effect which those rays themselves 

 produce upon the particles of air on which they impinge. The pro- 

 perty of radiation, however, is that which chiefly concerns the horti- 

 culturist ; and the following description of this phenomenon is given 

 by Mr. Daniell, the author of by far the best essay which has yet 

 appeared on climate, as connected with horticulture. 



Radiation of heat is the " power of emitting it in straight lines in 

 every direction, independently of contact, and may be regarded as a 

 property common to all matter. Co-existing with it, in the same 

 degrees, may be regarded the power of absorbing heat so emitted 

 from other bodies. Polished metals, and the fibres of vegetables, may 

 be considered as placed at the two extremities of the scale upon which 

 these properties in different substances may be measured. If a body 

 be so situated that it may receive just as much radiant heat as itself 

 projects, its temperature remains the same ; if the surrounding bodies 

 emit heat of greater intensity than the same body, its temperature 

 rises, till the quantity which it receives exactly balances its expendi- 

 ture, at which point it again becomes stationary ; and if the power of 

 radiation be exerted under circumstances which prevent a return, the 

 temperature of the body declines. Thus, if a thermometer be placed 

 in the focus of a concave metallic mirror, and turned towards any clear 

 portion of the sky, at any period of the day, it will fall many degrees 

 below the temperature of another thermometer placed near it out of 

 the mirror ; the power of radiation is exerted in both thermometers, 



