70 



HEAT, CONSIDERED WITH 



plained by Dr. Wells on these principles. The deposition of moisture is 

 owing to the cold produced in bodies by radiation, which condenses the 

 atmospheric vapour on their surfaces. The deposition of dew takes 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. Dr. Wells says that dew 

 is " never formed upon the good conducting surfaces of metals, but is rapidly 

 deposited upon the bad conducting surfaces of filamentous bodies, such as 

 cotton, wool, &CC." There would appear to be some mistake in the assertion, 

 that dew is never formed on metals ; for any one may prove the contrary by 

 breathing on the blade of a knife. It is true dew is seldom found on bright 

 surfaces, such as metals or glass, in the form of drops, as it is on rough and 

 pointed objects like wool, grass, &c. ; but there can be no doubt of its exist- 

 ence on these bodies, though in a less conspicuous form. W ere this not the 

 case, the law of the deposition of water from air w^ould not be universal. 

 This law is, that moisture, or deposition of moisture, including that modifi- 

 cation 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. 



224. In remarking that dew is never formed upon metals^ Mr. Daniell 

 observes, " it is necessary to distinguish a secondary effect which often causes 

 a deposition of moisture upon every kind of surface indiscriminately. The 

 cold w^hich is produced upon the surface of the radiating body is communi- 

 cated by slow degrees to the surrounding atmosphere ; and if the effect be 

 great and of sufficient continuance, moisture is not only deposited upon the 

 solid body, but is precipitated in the air itself ; from which it slowly sub 

 sides, and settles upon everything within its range. 



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

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

 densed, 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. A compensating arrangement is 

 thus establislied, which, while it produces all the advantages of this gentle 

 effusion of moisture, guards against injurious concentration of the cause by 

 which it is produced. ' 



220. " The effects of radiation come under the consideration of the hor- 

 ticulturist in two points of view : the first regards the primary influence 

 upon vegetables exposed to it ; the second, the modifications produced by it 

 upon the atmosphere of particular situations. To vegetables growing in the 

 climates for which they were originally designed by nature, there can be no 

 doubt that the action of radiation is particularly beneficial, from the depo- 

 sition of moisture which it determines upon their foliage : but to tender 

 plants artificially trained to resist the rigours of an unnatural situation, this 

 extra degree of cold may prove highly prejudicial. It also appears probable, 

 from observation, that the intensity of this action increases with the distance 

 from the equator to the poles ; as the lowest depression of the thermometer 

 which has been registered between the tropics, from this cause, is 12°, 

 whereas in the latitude of London it not unfrequently amounts to 17°. But 

 however this may be, it is certain that vegetation in 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 



