SOMETHING NEW ABOUT THE FORMATION OF DEW.

Daily Telegraph (Napier), Issue 3018, 26 February 1881, Page 4

SOMETHING NEW ABOUT THE FORMATION OF DEW.

Wβ are all familiar with the bright, transparent beads of water formed upon grass and other kinds of vegetation during the clear and still nights of summer and autumn. This water is known as " dew," and the generally received theory of its formation was first clearly set forth by Dr. W. C. Wells, a physician of London, in his famous X Essay on Dow " first publishod in 1814. This essay still continues to be the standard authority on the subject, and so far as the writer is aware, nothing further new or of special iuterest in this connection has found its way into any standard publication up to the present time, Dr Wells' theory, as is well known, sets forth that dew is a deposit of invisable moisture from tho atmosphere upon the surfaces, the temperature of which is lower than that of the surrounding atmosphere. . The deposit formed in a warm clay upon the outer surface of a pitcher of cold water is due to the same law. But now comes Mr J. TJ. Lloyd, with a series of articles in tho Christian Standard, of Cincinnati, commencing with July 24th 1880, in which he admits that the idea is ' beautiful and the theory true to a certain extent, but denies that ail the moisture thus observed is derived from the atmosphere. Hβ holds that there are two kinds of dew, derived from entirely unlike sources, and which, when separately collected, show entirely different chemical characteristics. The one, when deposited from a pure atmosphere, is almost pure water, while the other, although collected from vegetation exposed in the same field and to the same atmosphere as the firsb contains so _ large an amount of sugar (glucose) as, in some instances, to be quite ■perceptible to the taste. Mr Lloyd says he had noticed that on certain species of grass, dew is found in the evening in drops upon the tip end of each thrifty blade, before the remainder of tho blade is moistened, and before there is moisture upon the surface generally of any vegetation in the vicinity ; that these drops are continually falling off, from their accumulating weight, and are constantly being replaced by other drops at the same point. It was evident that these drops were not condensed from the atmosphere.. He noticed that while one species first shows the drow upon the tip of the blade, others were covered ■with beads of moisture simultaneously along the entire edge of the blades, while still other herbs and leaves remained perfectly dry or exuded moisture from their entire surface. He asks, " Can this variation be explained by any law of radiation ?" and answers the question in the negative. He argues, both from observation and reason, that dew proper ia deposited from the atmosphere, according to Dr Wells' theory, but that the drops formed at the tips arid edges of certain grass blades and leaves as above are exudedfrom the plants. The sugar found

in such drops is tlio surplus of that which, is formed in tho upward passage of the fluid, and which not being appropriated by the plant, goes to waste in solution with the exuded water. Hence the familiar fact that cattle prefer grass which is or has recently been wet with dew (exuded moisture) 5 it is sweeter than that which is more dry. The term "honey dew" is frequently applied to a moisture which sometimes attracts beos and other insects by its peculiar sweetness, Thore seems to be a great difference in the amount of water excluded by different plants. " Elite grass " is especially noticeable for the rapid formation of drops on the tips of its leaves. The plant commonly known as the " Indian turnip," the large leaved species of Caladium, "or "Elephant's ear," etc., are also specially noticeable. He also holds that this excreted dew is greater in quantity than that which is condensed from !Vj i-.fmcnphero. Ths philosophy of this exudation may be condensed from one of the Doctor's papers, as follows :—The rootlets of each clump of grass, or of each shrub or bush, are constantly absorbing from the earth water charged with such mineral ingredients as are needed for the growth of the plant, and th 0 grass blades and leaves are continually exhaling the surplus water into the atmosphere. The cells of the plants seize upon the nutritive principles, appropriating them to their support and growth. The surplus, or depleted water, escapes in the day time by insensiblo evaporation; but during the night, when but little or no evaporationtakes place, this moisture accumulates upon the leaf surfaces and becomes visible. In some plants it exudes mainly from a feyr comparatively large microscopic openings in the tip of the leaves, as in blue grass ; from others along the edges ; from others still, I evenly over tho entire surface. The latter, 1 for evident reasons, is more generally con--1 fined to wooden plants with leaves and branches. These deductions, the Doctor avers, are the result of extended observations, sometimes carefully conducted with, chloride of calcium and bell glasses, and by other means. The question here raised is certainly a very interesting one, and will no doubt be speedly confirmed or exploded. It is given in these columns for what it is worth as an interesting item in the progress I of scientific investigation.