Nitrate of soda,[202] or, as it is more correctly designated from a chemical point of view, sodium nitrate, now forms the chief artificial nitrogenous manure in use. Along with sulphate of ammonia, it has taken the place once held in the manure markets by the older Peruvian guano, and may without doubt be reckoned, at present prices, one of the cheapest and most valuable of the artificial sources of nitrogen for the plant. It is some sixty-two years ago since it was first exported from South America into this country. The total exports in that year amounted to about 800 tons, and some indication of the enormous extent to which the use of this valuable fertiliser has been developed since then will be obtained from the statement that the total exports at present amount to little less than 1,000,000 tons per annum, representing a monetary value of 6 to 7 millions sterling. Of this quantity about 120,000 tons are imported into Britain.[203] While its chief use is for manurial purposes, it must not be imagined that it is only used for this purpose. A certain amount is used in connection with various chemical manufactures--for instance, that of nitric and sulphuric acid--and also in the manufacture of saltpetre, the chief constituent of gunpowder. _Date of Discovery of Nitrate Deposits._ The exact date of the discovery of the nitrate deposits seems to be a point of considerable dubiety. The earliest published description of them was written by Bollaert about the year 1820, in which year, it is stated, the first shipment was made to England. It was not, however, till some ten or twelve years later that the Peruvian Government, to whom they then belonged,[204] seems to have recognised their value. The most important deposits are found in the vicinity of the town of Iquique, which is the chief nitrate port of South America. It is a somewhat striking fact that this substance, which has conclusively proved itself to be the most potent of all known artificial agents in the promotion of vegetable growth, should be found in a district utterly lacking the slightest traces of vegetation of any kind. Lest such a statement should seem to savour of irony, we hasten to explain that the singular barrenness of this part of the country is largely due to the character of its climate, the deposits occurring in the midst of sandy deserts,[205] on which rain never falls. _Their Origin._