The theory is very evident. The mixed nitrate, consisting of variable proportions of nitrate of lime and nitrate of potassa, is extracted from the saltpetre earth by water, which dissolves it. Now, as the affinity of nitric acid for potassa is greater than for lime, and consequently potassa will decompose nitrate of lime, when the lixivium is passed through wood ashes, the potassa they contain will unite with the nitric acid, and the lime be separated, which remains in the hopper. The liquor holds in solution no other salt than nitrate of potassa, provided the quantity of alkali in the wood ashes be sufficient to effect the decomposition;--if _more_, it will pass through in an uncombined state; and if _less_, the liquor will contain nitrate of lime. As the alkali contains more or less carbonic acid, the decomposition is not a case of single but of double affinity, in which we form, at the same time, a carbonate of lime. When the solution is boiled, and set aside in the troughs to crystallize, the nitre will form in a regular manner. The mother water, or the fluid which remains after the crystallization, may contain, from the circumstance before stated, either potash, or undecomposed nitrate of lime--hence it is thrown on the hopper in a subsequent operation. The nitre, however, as made at the caves, is called _rough_ or crude nitre. Before it is used for the manufacture of gunpowder, and other purposes, it is purified or refined. This operation, which we shall notice more fully hereafter, is nothing more than the separation of all earthy salts, and the alkaline muriates and sulphates; in other words, the conversion of the whole by the separation of foreign substances, into pure nitrate of potassa. The mode of treating the _rock ore_, or sand rocks, which contain nitre, is the same as before given. It contains more nitrate of potassa, and therefore requires less potash, and in some instances, the nitre is perfectly pure. The sand rocks often yield twenty or thirty pounds per bushel. A mass of pure nitre, weighing sixteen hundred pounds, has been discovered. Smaller masses have also been found. The rocks which contain the greatest quantity of nitre are extremely difficult to bore, and are tinged brown or yellow. Saltpetre makers find it to their interest to work the rock ore in preference to the calcareous nitrate, as it yields more nitre. It is a fact well known, that foreign saltpetre contains a variety of deliquescent salts, or those salts which attract and absorb moisture and also common salt. The efforts of European refiners are directed to their separation. The saltpetre of the Western country, Dr. Brown assures us, does not contain common salt.