The Surface Tension of Water. — Ladd. 277 
a cause. In a paper by Alfred M. Mayer,* on "The Floating 
of Metals and Glass on Water and Other Liquids," it is stated 
as a result of experiments that materials which will ordinarily 
attract water, may fail to do so when films of air are con - 
densed on their surfaces, with the result that they will float 
upon the liquid in question, even when loaded by weights 
exceeding in value that of the "surface tension " of the 
given liquid. 
If, now, the grains of sand on the surface of the dune be 
in the condition of the material described above, the attraction 
between them and the water fails, at least temporarily, to oper- 
ate. The falling rain drops, larger than the interstices of the 
sand, immediately coalesce into a sheet, which might remain, 
through the operation of the tension on its lower surface, a 
body of water of appreciable thickness supported on the col- 
umns of sand penetrating it from below, but not in actual 
contact with it. (See fig.III.) 
Prof. Whitney says, in "Conditions in Soils of the Arid 
Region," year book, U. S. Department of Agriculture, 1894, 
p. 160: 
"Water descends very slowly and to a very limited extent in a 
perfectly dry soil * * * because the tension or contracting power 
of the surface of the water is greater than the attraction of the soil 
grains, which tends to cause its dififusion through the mass." 
This observation agrees with that made by Prof. Shaler 
in the field, and with the result of Mr. Mayer's laboratory ex- 
periments. Nevertheless, in the experiments conducted by the 
writer, c-/<?j/ dried to constant weight at 100^ C. was used, with 
the result that it absorbed water with extraordinary rapidity 
and copiousness (in cases, as high as 200 per cent by weight, 
as stated above). This would seem to indicate that some other 
condition must be fulfilled than mere dryness, to render the 
material non-absorptive of water. In fact, while making 
chemical analyses of clays, and while determining hygroscopic 
moisture. I have observed that the dry, clay powder would ac- 
tually gain weight while in a chloride of lime desiccator, even 
when left there for hours. It did not, however, so gain when 
sulphuric acid replaced the chloride of lime. 
*Science' N. S., Vol. IV, No. 88, Sept. 4, 1896. 
