_Magnesia._—This is precipitated by ammonium sodium phosphate, filtered, ignited, and weighed as pyrophosphate. The magnesium is then calculated as chlorid. Magnesia is rarely found in excess of one-fourth per cent. When this amount is not exceeded the estimation of it may be neglected without any great error. As has already been said the chlorin is all calculated as sodium chlorid. If a part of it be combined with one-fourth per cent of magnesia it would represent 0.59 per cent of magnesium chlorid instead of 0.73 per cent sodium chlorid. In omitting the estimation of the magnesia therefore the importer is only damaged to the extent of 0.14 per cent of sodium nitrate. _Sodium Iodate._—This body, present only in small quantities, may also be neglected. In case the content of this body should reach one-fourth per cent the estimation of chlorin by titration using potassium chromate as indicator is impracticable. Such an instance, however, is rarely known. _Approximate Results._—When the determinations outlined above have been carefully made it is claimed that the result obtained by subtraction from 100 will not vary more than from two-tenths to three-tenths per cent from the true content of sodium nitrate. The method, however, cannot be considered strictly scientific and is much more tedious and chronophagous than the direct determination. In the direct determination, however, the analyst must assure himself that potassium is present in only appreciable quantities otherwise the per cent of sodium nitrate will be too low. The presence of potassium nitrate is a detriment in this respect only; _viz._, that it contains a less percentage of nitrogen than the corresponding sodium salt. As a fertilizer, the value of Chile saltpeter may be increased by its content of potassium.