In mineralogical literature there appears to be a tendency to overestimate the degree of exactness attainable in the chemical analysis of minerals. When it is considered how impossible in the majority of eases it is to obtain absolutely pure material for analysis, and how imperfect even in these days are some of the methods of chemical separation employed, for the determination of a formula any great reliance in the figures to the right of the decimal point in the percentages obtained is scarcely justified in the case of analyses made on small amounts (less than half a gram) of material.