134 WILLIAM H. HESS 
leaves its load of nitrate behind as an incrustation on the sur- 
face, while water from the interior of the pile works gradually 
towards the surface to take the place of the water removed by 
evaporation. Thus the materials soluble in water are slowly 
brought to the surface and left as a deposit which may be 
removed mechanically. This is an old method of obtaining 
saltpeter from manure heaps, and it is even now used to a small 
extent in Burope, siihey occurence) of the smitrates; imecayesmas 
an incrustation on the surface of the cavern earth shows that 
water has been removed by evaporation in much the same way 
as from the overhanging cliff and from the compost heap. 
We always have nitrogenous matter scattered over the sur- 
face of the soil and this decaying vegetation furnishes contin- 
uously during .its decay a small amount of nitric acid. All 
nitrates are soluble in water and so are sure to be found in the 
percolating water. If, then, the percolating water is intercepted 
and evaporated, the nitrate must be left behind. Nitrates should, 
therefore, occur in all caves and analyses of the cavern earths of 
a great number of caves in Indiana and Kentucky demonstrates 
that the occurrence of nitrates in cavern earths is general. No dry 
cavern earth was found which did not contain soluble salts of 
nitric acid, and these salts were distributed uniformly from the 
entrance to the end of the cavern. 
WituiaMm H. HEss. 
February 23, 1900. 
