INTRODUCTION.

In my treatise entitled, "Elementary Instruction in Qualitative Chemical Analysis," I have stated that ANALYTICAL CHEMISTRY Comprehends two branches, viz., "qualitative analysis," and " quantitative analysis," and that the object of the former is to ascertain the individual kind and nature, that of the latter to determine the absolute and relative quantity or proportion of the several component parts of any given compound. By QUALITATIVE ANALYSIS we convert the unknown constituents of a given compound into certain forms or combinations with the properties of which we are acquainted; and we are thus enabled to draw correct inferences respecting the nature of every individual constituent of the compound in question. Quantitative analysis attains its object, according to circumstances, often by very different ways; the two methods most widely differing from each other, are "analysis by weight," or gravimetrical analysis, and "analysis by measure," or "volumetrical analysis."

QUANTITATIVE ANALYSIS BY WEIGHT, or GRAVIMETRICAL ANALYSIS, has for its object to convert the known constituents of a given compound into such forms or combinations as will admit of the most exact determination of their weight, and of which, moreover, the relative and combining proportions are accurately known. These new forms or combinations may be either educts from the analysed compound or mixture, or they may be products. In the former case we have simply to weigh the eliminated substance, since its ascertained weight, of course, supplies at once the direct expression of the quantity, or proportion in which it existed in the compound under examination; whilst in the latter case, that is, when we have to deal with products, the quantity, or proportion in which the eliminated constituent was originally present in the analysed compound, has to be deduced by calculation from the proportion in which it exists in its new combination.

The following example will serve to illustrate these points :-Suppose we wish to determine the proportional amount of mercury contained in the chloride of that metal; now, we may do this, either by precipitating the metallic mercury from the solution of the chloride, by means of protochloride of tin; or we may attain our object by precipitating the solution of chloride of mercury by sulphuretted hydrogen, and weighing the precipitated sulphide of mercury. 100 parts of chloride of mercury consist of 73.83 of mercury and 26.17 of chlorine; consequently, the precipitation of a solution of chloride of mercury by protochloride of tin will yield 73.83 per cent. of metallic mercury, which may be verified by simply weighing the precipitated metal. The