The Origin of the Nitrates in Griqualand West 127 
salt, it creeps by efflorescence to the projecting points of the rocks, and 
accumulates in the soil and on the rock at the base of the cliff. The 
nitrate of calcium, however, being more soluble in water than the 
nitrate of potash, would not be separated from it by this natural refining. 
That is done by a secondary chemical action. 
In all the samples of nitrate-bearing soil* which I analyzed, I found 
a considerable amount of sulphate of lime; and in a few localities tooth- 
like pieces of saltpetre and gypsum occurred close together on the rock 
just near the surface of the soil. The peculiar form of these specimens 
proves that both were formed very slowly on the spot by the double 
decomposition of nitrate of calcium and sulphate of potassium, the 
sulphur being derived from the albuminvids of the refuse matter. 
Most samples of apparently pure saltpetre, even if free from 
mechanical impurities like clay or sand, still contain nitrate of lime, and 
traces of sulphate of lime and chloride of sodium ; but a few colourless 
transparent specimens which I found are chemically pure nitrate of 
potassium. 
From the preceding description, it is apparent that the occurrence of 
saltpetre in this region has no resemblance whatever to the nitrate-beds 
of South America, where the saline material (chiefly nitrate of soda) 
forms layers many feet thick, and spreads over hundreds of square miles. 
Nor is it like the deposits in North America, Venezuela, and several 
other tropical countries, where the remains of antediluvian animals, 
accumulated in caves in layers ten to thirty feet deep, have undergone 
nitrification and formed nitrate soils, which principally contain the 
nitrates of potassium, calcium, and magnesium. 
To a certain extent it may be compared with some localities in India 
and Ceylon, where the walls of caves filled with bat guano gradually 
become incrustated with nitre. 
A much more exact comparison, however, is offered by the artificial 
manufacturing of saltpetre as it was formerly carried out in Southern 
Europe. In the so-called saltpetre-gardens animal refuse and waste- 
products were spread over soil mixed with lime, and thus exposed to 
the nitrifying agents. The first product of this operation was nitrate 
of calcium, which was decomposed into nitrate of potassium by the 
addition of pearlash—processes almost identical with those described 
above. 
But this natural chemical factory not only produces the crude 
* Composition of a sample of nitrate-soil : 
Potassic nitrate ...  —.... soa SRR} Organic matter ... soy MOSK 
Calcic nitrate... sot Bean pe eeeAs) Insoluble in water after a} 34-1] 
Magnesic nitrate Br pee elt a: (oxide of iron, rae Be 
Calcic sulphate ... she ee ESO Moisture ... eels 9 
Sodic chloride ... ee eh 14 Not Gene nee ae 2°18 
