EVAPORATION— CONDUCTION. 16t 



^er'e are ttree perfectly distinct modes in wLich. tlie surface Of tW 

 earth becomes cooled, and these are by evaporation, by conductionj and 

 by radiation. When water evaporates it becomes colder, because, in 

 the formation of vapour, heat is always absorbed. This simple fact is 

 of the greatest importance to the Hfe of both plants and animals* 

 When plants are exposed to a hot sunshine, the moisture which they 

 contain gradually evaporates, and in so doing absorbs the great heat of 

 the sun's rays, which would otherwise injure plants and bum them up. 

 Evaporation from the surface of the leaves is generally in proportion to 

 the direct heat of the sun, and it is necessary as a means of keeping the 

 plant cool, and preventing it from being scorched ; if soil is dry, so that 

 the plant cannot obtain, by means of its roots, a constant supply of 

 moisture to keep up this daily evaporation from its leaves, it has no 

 power of withstanding the heat of the sun, and it withers and fades the 

 first hot day. Whenever, and ia whatever manner, we check the 

 constant evaporation which always goes on in the leaves of a healthy 

 plant, we run a risk of killing it by exposure to hot sunshine. The 

 common experience of the gardener gives plenty of examples of the 

 truth of this; but there are other eases in which, though the same effect 

 is produced, and the same principle is involved, its influence is not so 

 self-evident. When, for example, a plant is placed in a close hothouse, 

 confined in a hot damp air, its perspiration is cheeked, because the air 

 being already saturated with moisture, it has little power of carrying 

 off the moisture evaporated by the leaves, and consequently the plant 

 has less power of withstanding the heating influence of the sun's rays 

 than it has in the open air, or in a state of nature. 



As evaporation, on the oiie hand, is a natural means of counteracting 

 the excessive heat of the sun, so, on the other hand, it is the chief 

 cooling agent in nature, and every ciroumstance tending to increase 

 evaporation from the surface of the soil tends also to cool it. As a 

 moist air and a diminished circulation are most unfavourable to evapora- 

 tion, so a dry air and free circulation grea/tly facilitate it.. The cooling 

 effect of a cold dry wind is familiar to every one ; its iofluenee depends 

 on the fact, that dry air readily absorbs moisture from any surface 

 exposed to it, whilst the rapid motion of the wind, by carrying away 

 the moisture as fast as it is formed, insures a constant supply of fresh, 

 dry air, and thus, by aiding in the formation of moisture, rapidly cools 

 the surface on which it blows. 



The second mode in which plants are cooled is by conduction, or by 

 the mere contact of cold air ; and this is quite independent of the cold 

 produced by evaporation. When a cold wind drives along the surface 

 of the ground it gradually cools it, and, of course, likewise the plants 

 growing on it, by the simple abstraction, or carrying away of heat. S» 

 long as the surrounding air is colder than the plants it wiU tend to 

 reduce their temperature ; and if the air is in motion, as fresh portions 



