February 2, 1885.] 



THE TROPICAL AGRICULTURIST. 



599 



DEW. 



BY R. PUR YE All, LI..D., PBOFESSOIi OF CHEMISTRT i\ 



KKHMOND COLLEOE. 



linv has long been a favorite theme alike in the realm 

 of fancy ami of science. Poetry has characterized the dew 

 drops, sparkling in the morning suu on grass and Uower, 

 as the tears which pitying angels shed upon a sorrowing 

 earth. Science explains how and when and why the glitter- 

 ing spherules are deposited, and, in doing so, robs the 

 subject of its sentimental and poetic interest. Abundant 

 compensation, however, is made in the new features and 

 added charms with which science invests the subject. 

 Nothing is simpler than the formation of dew. Indeed, 

 we make it every day. A pitcher containing ice water 

 soon has its outer surface coated with drops of water, 

 and these little drops of water are really dew drops. What 

 has happened? Only this: the chilled surface of the \ 

 has robbed of its heat the aqueous vapor of the surround- 

 ing atmosphere, and so has condensed it into water. Dew 

 is formed in nature in precisely the same way, the only 

 difference being in the mode of obtaining chilled surfaces. 



Let us see. The atmosphere is always charged, more 

 or less, with aqueous vapor. The earth is always throw- 

 ing off heat into free space by the process of radiation. 

 During the day, however, the earth receives, as a general 

 statement, more heat from the sun than it loses by radiation. 

 Its temperature therefore rises during the day. At night, 

 however, heat being lost by radiation, and none being 

 received from the sun, the temperature of the earth's surface 

 and of all bodies on the earth's surface sinks. Now the 

 atmosphere brooding over surfaces thus chilled at night by 

 radiation itself becomes chilled; and its aqueous vapor, 

 losing heat, is deposited upon the chilled surfaces as water 

 or dew'. 



What are some of the leading circumstances that modify 

 the deposit of dew? 



(1.) The hygrometric condition of the atmosphere. 

 When the atmosphere is saturated with aqueous vapor, 

 then the slightest reduction of temperature will cause a 

 deposit of dew; and the deposit will be very copious if 

 there be a large reduction of temperature. Hence, as we 

 walk out in the morning now, during the present distress- 

 ing drought, we find but little dew on the grass. The 

 explanation is obvious. The air is arid, and hence, though 

 all other circumstances favorable to the formation of dew 

 exist, the deposit is small. The material for forming 

 dew — aqueous vapor — is scant, and hence the scanty deposit 

 of dew. 



(2.) We notice a remarkable difference in the amounts 

 of dew deposited on different objects. When there is a 

 moderate deposit of dew, some objects, as living veget- 

 ation, have quite a good supply, while other objects, as 

 dead matter of every sort, have little or none. When the 

 supply of dew is copious, we find the grass literally dr 

 with it, while there is largely less on the rocks and fences 

 and dead vegetation. Those objects that need dew have 

 a corresponding power of causing its deposit, while those 

 that do not need it show a corresponding lack of power. 

 In other words, all objects have the inherent power of 

 causing the aqueous deposit in the exact proportion of 

 their wants, so that when the supply of dew is moderate, 

 the whole or nearly the whole of it is deposited upon such 

 objects as need it. Those that do not need the dew receive 

 it only when the deposit is extremely copious, and nature 

 can afford, as it were, to be prodigal in the bestowal of 

 her gifts. Why this wonderful difference in the capacity 

 of different bodies to cause the deposit of dew? A word 

 explains. Different bodies lose heat very unequally by 

 radiation, and of all bodies, that power is greatest in living 

 vegetation. Grass and growing vegetation of every kind, 

 losing heat by radiation much more rapidly than dead matter, 

 suffer of course during the nighl a much greater reduction 

 of temperature, and hence a correspondingly larger amount 

 of dew is deposited upon them. But for exigent duty, 

 we might stop to moralize on this beautiful and wonderful 

 provision, which gives to every waving leaflet the power 

 of abstracting from the atmosphere the water which it 

 needs. 



(3.) Another circumstance which modifies the deposit 

 of dew is the clearness ai 1 stillness of the atmosphere. 

 Other things being equal, the deposit will be lar 



clear than on cloudy nights. Why? When clouds over- 

 hang the earth, they reflect back tin- heat and so prevent 

 its dispersion in free space. Hence the sultry, stifling 

 character of cloudy nights in hot weather. Heat being 

 thrown back to the earth by the overhanging canopy of 

 clouds, the earth and objects upon the earth are chilled 

 but little, if at all, and lunce the deposit of dew is small. 

 Nor can the deposit be abundant unless the air be still. 

 • '.insider a meadow of green grass rapidly radiating its 

 heat into space during the clear, still night. The grass 

 soon becomes chilled to the dew point and the deposit 

 is made. But if the weather be windy, the air cannot 

 brood over the chilled grass long enough to have its 

 temperature reduced sufficiently for the condensation of 

 the aqueous vapor of the atmosphere. Hence, on windy 

 or cloudy nights, how favorable soever other circumstances 

 may lie, the deposit of dew is always small. 



When frost, which is frozen dew. is threatened, the 

 farmer, particularly the tobacco grower, is relieved of all 

 his fears if at nightfall either a good breeze springs up 

 or clouds begin to form. 



In the above we showed that the deposit of dew is most 

 abundant on those objects that need it most. By another 

 wonderful and beneficent arrangement, the deposit is most 

 abundant when it is most needed . W hen is dew most needed ? 

 We auswerin the spring and fall ; particularly, in this latitude, 

 in the months of April and May, and September and 

 October. Why? In April and May, seeds are planted 

 that are to fructify in the summer and autumn. They 

 are covered slightly, only a few inches, sometimes barely 

 one or two. They germinate in the warm and humid soil, 

 and soon appear above the surface. But the young root- 

 lets are so near the surface at first that a few dry days 

 would so exhaust the surface soil of moisture that the 

 plants would perish. Before the roots have struck down 

 deeply into the permanently moist soil, and when they 

 must rely therefore upon the first few inches ofsurface 

 soil for a scant and precarious supply of water, kindly 

 nature, mindful of the situation, every night gives the baby 

 plants a little extra pap in the form of dew. 



It is obvious then that during the first days and weeks 

 of plant life, dew is most needed, because at that time 

 the rootlets must get their supply of water from near the 

 surface, which is liable to become dry by only a day or 

 two of hot, windy weather. A supply of water, extraneous 

 to tli. soil, is needed, and that supply comes every night 

 from the condensation of the aqueous vapor of the atmo- 

 sphere. That the supply of dew is vastly more abundant 

 in the spring and fall, when it is most needed, than it is in 

 midsummer, when the need is far less imperative, is too 

 familiar and obvious a fact to have escaped the notice even 

 of the least observant. 



Now, let us look iuto the cause of the fact; let us see 

 the operation of these agencies, which give us abundant 

 deposits of dew at those seasons of the year when veget- 

 ation most demands it, and diminish the supply when the 

 demand is diminished. 



Why should the deposit of dew be so much more abundant 

 in April and May than in July and August ? We will 

 recur briefly to the mode of the formation of dew. Objects, 

 to have dew deposited upon them, must become sufficiently 

 chilled at night, by the radiation of heat, to condense 

 the aqueous vapor of the atmosphere. All objects must 

 be reduced in temperature to the dew point. This is much 

 more easily accomplished jn May than in August for two 

 obvious reasons. In May the sun is not so hot, nor does 

 it shine so long, as in August. The temperature of the 

 atmosphere and of the earth's surface is not so much 

 elevated in the day in May as in August. To get dew 

 then objects have to sink at night to the dew point from 

 a much lower temperature in May than in August. In 

 the latter month, they must sink from eighty or ninety 

 degrees, in the former, from sixty or seventy degrees, to 

 the dew point. In order therefore to get a copious deposit 

 of dew, plants have less work to do in spring or fall than 

 in summer; but not only so, they have more time to do 

 it in. In May and September, the nights are longer than 

 in July and August, and hence plants have more time to 

 into space. In the spring and fall, plants, 

 in order to get dew. have less work to do anil more time for 

 doing it'than in summer; and hence the abundant deposits 

 in the spring and fall, and the scant supply in midsummer. 



