_The Method of mining the Nitrate._ The mode in which the _caliche_ is excavated is as follows: A hole is bored through the _chuca_, _costra_, and _caliche_ layers till the _cova_ or soft earth is reached below. It is then enlarged until it is wide enough to admit of a small boy being let down, who scrapes away the earth below the _caliche_ so as to form a little hollow cup. Into this a charge of gunpowder is introduced, and subsequently exploded. The _caliche_ is then separated by means of picks from the overlying _costra_ and carried to the refinery. _Composition of Caliche._ Both in appearance and composition it varies very much. In colour it may be snow-white, sulphur, lemon, orange, violet, blue, and sometimes brown like raw sugar. The _caliche_ found in the Pampa de Tamarugal contains generally about 30 to 50 per cent pure nitrate of soda; that in the province of Atacama contains from 25 to 40 per cent. The subsequent refining processes, which consist in crushing it by means of rollers and then dissolving it, need not here be described. It may be sufficient to mention that the process used is that known as systematic lixiviation, and is analogous to the method introduced by Shanks in the manufacture of soda. The chief impurity in the raw material is common salt: gypsum, sulphates of potassium, sodium, and magnesium, along with insoluble matters, are the other impurities. The manufacture of iodine, which, as has been already noticed, is found in the nitrate-beds, is also carried on at these _oficinas_. _Extent of the Nitrate Deposits._ The question of the extent of the nitrate of soda deposits is naturally one of very great interest, especially from the agricultural point of view. M. Charles Legrange, a French writer, estimated a few years ago that they still contained about 100,000,000 tons of pure nitrate of soda. Opinions on this point differ very considerably, and it seems wellnigh impossible to arrive at any very accurate estimate. The number of years they will last will depend, of course, on the amount of annual exportation. This, at present, falls little short of 1,000,000 tons. If this amount is maintained, they should last, according to experts, some twenty or thirty years at least. A consideration which has an important influence on this question, is the price obtained for the article. If this should be increased, it may be possible to treat the larger quantities of the inferior raw material (which at present prices are allowed to accumulate) at a profit. Undoubtedly this is what will ultimately take place, when the richer quality of the _caliche_ has been exhausted. _Composition and Properties of Nitrate of Soda._