The Adsorption of Water Vapor by Fibers Has Been Studied Experimentally by Very Many

J. Chem. Ed., 39, 584 (1962)

Gideon’s fleece tests

The Earliest Recorded Vapor Phase Adsorption Experiment?

C. H. Giles

The Royal College of Science and Technology

Glasgow, C.1, Scotland

The adsorption of water vapor by fibers has been studied experimentally by very many investigators in recent years, but the earliest recorded experiment on this phenomenon isprobably Gideon's pair of tests with a fleece (c.a. 1100 B.C.), reported as follows1:

"And Gideon said unto God, 'If thou wilt save Israel by mine hand, as thou hast said, behold, I will put a fleece of wool in the floor; and if the dew be on the fleece only, and it be dry upon all the earth beside, then shell I know that thou wilt save Israel by mine hand, as thou has said.' And it was so: for he rose up early on the morrow, and thrust the fleece together, and wringed the dew out of the fleece, a 'bowl full of water. And Gideon said unto God, 'Let not thine anger be hot against me, and I will speak but this once: let me prove, I pray thee, but this once with the fleece; let it now be dry only upon the fleece, and upon all the ground let there be dew.' And God did so that night: for it was dry upon the fleece only, and there was dew on all the ground."

A simple physical explanation exists for these effects which appear remarkable at first sight. The subject has been briefly discussed by Brunt,2 who pointed out that a fleece, being over 90 per cent air, has extremely low thermal conductivity and heat capacity, so that its rate of fall of temperature will be so high that its temperature may fall below the dew point of the atmosphere while the surrounding ground is still above.

The wool fibers in a fleece expose a very large surface to the air, and are consequently efficient dew collectors (as are the grains in sand dunes, which are so used in the Sahara). A fleece3 contains, per pound, about one million fibers (assumed dimensions 6.8 in. long X 0.002 in. diameter; specific gravity = 1.32). The total superficial area of fibers is thus about 300 ft2 per lb. Heavy dews occur throughout the year in Israel; deposits up to 3 mm have been recorded,. but 0.15 mm is an average figure.4 Even taking only the latter figure, and assuming that dew collects only on the outer 0.5-in. portions of the fibers (and that these are not matted together), a 10-lb fleece would collect about 0.7 gallons of dew, i.e., a total weight of about 7 lb of water. The fiber surfaces are largely hydrophobic (especially if the natural wool grease has not been completely removed), and the dew will therefore form drops which run down and collect as a mass of liquid water in the heart of the fleece. It is then protected against rapid evaporation, and will remain long after the dew on the surrounding ground has disappeared in the morning sunshine. Thus the result of the first test - a "bowl full of water" in the fleece with the surrounding ground dry - is readily explained.

The result of the second test can also be explained. Brunt2points out that in some circumstances water vapor diffuses upwards into the atmosphere through the surface of the ground, and that in the second test this water vapor would condense on cooled grass or stones, so that the ground would be damp, but the vapor would not penetrate the skin of the fleece.

A more detailed explanation is as follows: Dew deposits from the air only when a variety of fairly loosely defined conditions occur simultaneously, e.g., a clear sky, high humidity in surface layers of air, a moderate wind (2-6 mph), and a good radiating surface, thermally insulated. A slight change in any of these conditions can prevent true dew formation, but water droplets can still appear on grass or leaves. This "false dew" can be caused either by condensation of water vapor rising from the soil,5 or by "guttation," a process whereby drops of moisture resembling dew are exuded at the tips of grass or round the edges of leaves.6 Thus a slight change in meteorological conditions could have occurred during the night of the second test (perhaps a drop in wind speed), this could have caused "false dew," but no true dew, to form. If the conditions were such that no dew deposited on the fleece, this would remain "dry," while "false dew" would be seen on the surrounding grass. Even if a true but very light dew did deposit (provided it were not more than one-tenth of that of the preceding night, which would mean a weight of about 0.7 lb of water collecting in the 10-lb fleece), the water would nearly all disappear by adsorption into the internal structure of the fibers. If these were almost completely dry at the end of the first day, they could even adsorb up to about 5 lb of dew (i.e., 50% of their weight of water) during the following night without appearing wet to the touch. Leaves are of course already saturated internally with water and cannot adsorb more.

It is obvious that if the experimenter wished the results of these tests to be conclusive, on the first morning he would wait until the ground dew disappeared and then examine the fleece, and on the second morning he would examine it before dew disappeared.

Thus the phenomena described do not necessarily imply any departure from natural law. The unique feature is that the required meteorological conditions came at the exact times for each test to be successful.

I thank Mr. C. A. Halstead and Dr. N. Peacock for advice and discussions, and Dr. T. A. Margerison and Professor J. Mauchline for information.

1The Book of Judges, Chap. VI, vv. 36-40.

2BRUNT, D.,Proc. Royal Institution, London, 34, 57 (1947); Ann. Rep. East Malling Research Station, Kent, England, for 1958, p. 41 (1959).

3The fleece data are assumed to be average values for primitive sheep, see MAURSBERGER, H. R. (edit.), “Matthews’ Textile Fibers,” 5th ed., John Wiley and Sons, Inc., New York, 1947, chaps. 12-13.

4DUVDEVANI, S.,Quart. J. Royal Met. Soc., 73, 282 (1947).

5MONTEITH, J. L.,Quart. J. Royal Met. Soc., 83, 332 (1957).

6AITKEN, J.,Nature, 33, 256 (1886).