58 HEAT, CONSIDERED WITH 



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 stagnated 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 coverings, 

 whether of animals or plants, retain very little heat, when compared with 

 loose coverings ; and hence mats, when drawn tightly round bushes, or 

 nailed closely against trees on walls, are much less effective than when fas- 

 tened 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 litter or long straw. The heat of the trunks of trees is pre- 

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

 which is a powerful non-conductor (1^0), 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 Scotland, come nearly so early into 

 bloom. 



220. 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 transparent substances, and even polarised. 

 But though it be time that heat, in proceeding from a body, begins by radiat- 

 ing 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 proceed from 

 the uppermost part of the convex surface of the pipe will preserve their per- 

 pendicularity. 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 

 effects which those rays themselves produce upon the particles of air on which 

 they impinge. The property of radiation, however, is that which chiefly 

 concerns the horticulturist; and the following description of this pheno- 

 mena is given by Mr. Daniel], the author of by far the best essay which 

 has yet appeared on climate, as connected with horticulture. 



221. 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 



