At Corydon, in Indiana, there is a cave, which, according to Stilson's account, contains both nitrate of lime, and nitrate of magnesia. It is not worked. Kain, in his remarks on the Geology and Mineralogy of East Tennessee, (_Silliman's Journal_, vol. i, p. 65,) observes, that the numerous caves which have been found in the Cumberland mountains, and other parts of Tennessee have been very productive of nitrate of potassa; and in confirmation of the remarks before made, he adds, in investigating the causes that have given rise to these salts, that wild animals burrow in these caves; that, when pursued by the hunter, they make them the places of their retreat, and probably die there; that the aborigines have made them a place of burial; and that the streams of water, which flow through them, in wet weather, carry with them not only great quantities of leaves, but many other vegetable productions. Without offering any theory, by which we may account for the formation of nitre, in nitre caves, or in situations which cannot be influenced by the putrefactive process, we may merely remark, that as nitric acid is composed of oxygen and azote, there must be some operation unknown to us, by which the union of these elements takes place. Nascent azote must unite with the base of oxygen gas; but whence, in saltpetre caves, proceeds the azote and the oxygen? It appears that calcareous bodies facilitate the formation of nitre, as they do in artificial nitre beds. The greater part of the nitrous earth is lime; and it also appears, that the same earth, after the extraction of the saltpetre, will again furnish it. We know that lime is a compound of a base called calcium united with oxygen; but in what manner it promotes the union of azote and oxygen, or furnishes either one or the other of these bodies, or perhaps both, is altogether uncertain. Nor can we account for the formation of potash in the native nitre of the nitre caves. In other situations, as for instance where nitrous efflorescence appears on the earth, and in artificial nitre beds, in which animal and vegetable substances are in the act of decomposition by the putrefactive fermentation, we may account for the generation of nitric acid. It is extremely probable, that the azote of the atmosphere, and oxygen may combine spontaneously, under particular circumstances, in various operations of nature. Azote, it is known, forms with oxygen two gases, a protoxide and deutoxide, and the same elements in other proportions form nitric acid. Some condition, unknown to us, must, as an operating cause, produce this compound. As a condition for its generation, the presence of calcareous and alkaline matter, favours the formation of nitric acid. Of this fact, we have sufficient proof, in the generation of nitre in artificial nitre beds. But, with respect to natural causes, although the facts themselves are conclusive, we know little or nothing.