COLD PRODUCED BY EVAPORATION, AND ITS INFLUENCE. 43 



So science, from her daily converse with nature, fails not sooner or 

 later to take her hue and colour from the perception of this universal 

 love and bounty. Party and sectarian differences dwindle away and 

 disappear from the eyes of him who is daily occupied in the contempla- 

 tion of the boundless munificence of the great Impartial; he sees him- 

 self standing in one common relation to all his fellow-men, and feels 

 himself to be most completely performing his part in life, when he is 

 able in any way or in any measure to contribute to the general welfare 

 of all. 



It is in this sense too that science, humbly tracing the footstep? of the 

 Deity in all his works, and from them deducing his inteUigence and his 

 universal goodness — it is in this sense, that science is of no sect, and of 

 no party, but is equally the province, and the properly, and the friend of all. 



§ 7. Of the cold produced by the evaporation of water, and its 

 influence on vegetation. 



Beautiful, however, and beneficent as are the provisions by which, in 

 nature, watery vapour is made to serve so many useful purposes, there 

 are circumstances in which, and often through the neglect of man, the 

 presence of water becomes injurious to vegetation. 



The ascent of water, in the form of vapour, permits the soil to dry, 

 and fits it for the labours of the husbandman ; while its descent in dew 

 refreshes the plant, exhausted by the heat and excitement of a long 

 summer's day. But the same tendency to ascend in vapour, gives rise 

 to the cold unproductive character of lands in which water is present in 

 great excess. This character you are familiar with in what are called 

 cold clay soils. 



The epithet cold, applied to such soils, though derived probably from 

 no theoretical views, yet expresses very truly their actual condition. 

 The surface of the fields in localities where such lands exist, is in reality 

 less warm, throughout the year, than that of fields of a different quality, 

 even in their immediate neighbourhood. This is readily proved, by 

 placing the bulb of a thermometer immediately beneath the soil in two 

 such fields, when in the hottest day a marked difference of temperature 

 will, in general, be perceptible. The difference is dependent upon the 

 following principle : — 



When an open pan of water is placed upon the fire, it continues to 

 acquire heat till it reaches the temperature of 212° F. It then begins to 

 boil, but ceases to become hotter. Steam, however, passes off", and the 

 water diminishes in quantity. But while the vessel remains upon the 

 fire the water continues to receive heat from the burning fuel as it did 

 before it began to boil. But since, as already stated, it becomes no hot- 

 ter, the heat received from the fire must be carried off" by the steam. 



Now this is universally true. Whenever water is converted into 

 steam, the ascending vapour carries off much heat along with it. 



This heat is not missed, or its loss perceived, when the vapour or 

 steam is formed over a fire ; but let water evaporate in the open air 

 from a stone, a leaf, or a field, and it must take heat with it from these 

 objects — and the surface of the stone, the leaf, or the field, must become 

 colder. That stone or leaf also must beconis coldest from which the 

 largest quantity of vapour rises. 



