REFERENCE TO HORTICULTURE. 29 



place upon vegetables, but not upon the naked soil, because the latter 

 is a bad radiator as well as, a bad conductor of heat. The fibres of 

 short grass are particularly favourable to the formation of dew. 

 Moisture, including that modification of it called dew, is deposited 

 more or less on all bodies in absolute contact with the air, whenever 

 the temperature of the air is higher than that of the body with which 

 it is in contact. 



" The formation of dew is one of the circumstances which modify 

 and check the refrigerating effect of radiation ; for, as the vapour is 

 condensed, it gives out the latent heat with which it was combined in 

 its elastic form, and thus, no doubt, prevents an excess of depression 

 which might in many cases prove injurious to vegetation." 



" The vegetation of this country is liable to be affected at night from 

 the influence of radiation, by a temperature below the freezing point 

 of water, ten months in the year ; and even in the two months, July 

 and August, which are the only exceptions, a thermometer covered 

 with wool will sometimes fall to 35. It is, however, only low vege- 

 tation upon the ground which is exposed to the full rigour of this 

 effect. In such a situation, the air which is evolved by the process 

 lies upon the surface of the plants, and from its weight cannot make 

 its escape ; but from the foliage of a tree or shrub it glides off and 

 settles upon the ground." 



" Anything which obstructs the free aspect of the sky arrests in 

 proportion the progress of this refrigeration, and the slighest cover- 

 ing of cloth or matting annihilates it altogether. Trees trained 

 upon a wall or paling, or plants sown under their protection are 

 sheltered from a large portion of this evil, and are still further pro- 

 tected if within a moderate distance of another opposing screen." 



Almost all the modes in practice of protecting plants are designed to 

 check radiation, and hence the gardener should keep constantly in his 

 mind the fact, that all bodies placed in a medium colder than themselves 

 are continually giving out their heat in straight lines, and that these 

 straight lines, when the body is surrounded by air, may always be 

 reflected back on the body from which they emanate by the slightest 

 covering placed at a short distance from them, while, on the other 

 hand, if this slight covering is applied close to the body, instead of 

 reflecting back the heat, it will carry it off by conduction : that is, the 

 heat will pass through the thin covering closely applied, and be radiated 

 from its surface. Hence in covering sashes with mats, a great advan- 

 tage is obtained by laying straw between the mats and the glass, or by 

 any other means of keeping the mat a few inches above the frame. 

 Hence also when the branches of trees are to be protected by mats, they 

 will be rendered much more secure if first surrounded by straw, fern, 

 or some other light body which contains in its interstices a good deal of 

 air. " It should be borne in mind," Mr. Daniell observes, " that the 

 radiation is only transferred from the tree to the mat, and the cold of 

 the latter will be conducted to the former in every point where it 

 touches. Contact should therefore be prevented by hoops or other 

 means properly applied, and the stratum of air which is enclosed will, 



