Page:Popular Science Monthly Volume 85.djvu/143

Rh large to be of commercial value have been removed for use in the manufacture of fertilizers. Samples taken from some of the caves where the guano has undergone decomposition have been analyzed by the writer and found to run as high as 75 per cent, of potassium nitrate. Other samples have been examined which consisted of very pure elongated white crystals of calcium nitrate, and which had been taken from crevices in a cave. When placed in a humid atmosphere these soon melt in the moisture which they absorb from the air, but may be kept indefinitely in a desiccator.

While guano is usually considered to be the source of the nitrates found in caves, other theories are occasionally advanced to explain the origin of some of the deposits. Thus Hess considers that guano could not be the source of the large store of nitrates which have been taken from the Mammouth Cave at distances of over five miles from any opening which leads to the surface, since bats go, as a rule, but a short distance from the entrance to the cave. Moreover, in the bottom of many caves there are to be found earths from which nitrates can be extracted, but which do not contain any animal remains as would be expected if the nitrates were derived from guano. To explain the occurrence of nitrates in caves of this kind, the view is put forward that the nitrates do not come from guano, but originate in the surface soil above the eaves through the oxidation of organic matter by nitrifying bacteria. As the soil in limestone regions is usually loose and porous, the nitrates are considered to be carried down by percolating water and deposited in the floor of the caves. Air currents in and out of the caves would remove the water by evaporation, and the nitrates would consequently remain and would not be washed away so long as the inflow of water did not exceed that lost by evaporation.

A similar explanation is given for the origin of nitrate deposits under overhanging cliffs. Thus, water carrying nitrates dissolved from the soil percolates through the earth and a portion finally oozes out at the surface underneath where it evaporates and leaves the nitrates behind. Being protected from the rain in this position, the nitrates in this way are enabled to accumulate.

The theory advanced by Hess to account for the origin of nitrates in caves has not met with universal acceptance. Nichols has argued that bats do frequent remote parts of caves; that cave earth does contain organic matter; and that the proportion of phosphates to nitrates in the cave earth is much too great to be accounted for on the supposition that they were brought in by percolation from the surface soil. It is considered that while small deposits may be found in the way described by Hess, the great bulk of the nitrates that are found in caves results from