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Gravimetric methods are divided into three groups: (1) mechanical separations; (2) dry methods; and (3) wet methods. ~Mechanical Separations.~--Under this head are classed the method of assaying tin ores, known as vanning, and the amalgamation assay for gold. A set of sieves to determine the relative proportion of powders of different degrees of fineness is sometimes useful. A set with 10, 20, 40 and 80 meshes to the inch is convenient. ~Dry Assays.~--An important distinction between wet and dry methods of assaying is, that in the former the substance is got into the liquid state by solution, whilst in the latter fusion is taken advantage of. The difference between solution and fusion is easily illustrated: a lump of sugar heated over a candle-flame melts or fuses; suspended in water it dissolves. Many substances which are insoluble or infusible of themselves, become soluble or fusible when mixed with certain others; thus, in this way, solution is got with the aid of reagents, and fusion with the help of fluxes. For example, lead is insoluble in water, but if nitric acid be added, the metal rapidly disappears. It is convenient, but somewhat inaccurate, to say that the acid dissolves the lead. If the lead be acted on by nitric acid alone, without water, it is converted into a white powder, which does not dissolve until water is added; in this case it is obvious that the water is the solvent. The function of the acid is to convert the lead into a soluble compound.

One evening she was in difficulties about the quarantine, which was enforced that year on the Italian frontier. The local doctor had gone down that morning to see the Italian doctor and arrange some details. "Then, perhaps, my dear," she said to her husband, "he is the quarantine." "No, my love," replied her husband. "The quarantine is not a person, it is a place where they put people"; but she would not be comforted, and suspected the quarantine as an enemy that might at any moment pounce out upon her and her parrots. So a lady told me once that she had been in like trouble about the anthem. She read in her prayer-book that in choirs and places where they sing "here followeth the anthem," yet the person with this most mysteriously sounding name never did follow. They had a choir, and no one could say the church was not a place where they sang, for they did sing--both chants and hymns. Why, then, this persistent slackness on the part of the anthem, who at this juncture should follow her papa, the rector, into the reading-desk? No doubt he would come some day, and then what would he be like? Fair or dark? Tall or short? Would he be bald and wear spectacles like papa, or would he be young and good-looking? Anyhow, there was something wrong, for it was announced that he would follow, and he never did follow; therefore there was no knowing what he might not do next.

_Natural selection_ depends on the same three factors as _artificial selection_: on _variability_, _inheritance_, and _selection for breeding_, but this last is here carried out not by a breeder but by what Darwin called the "struggle for existence." This last factor is one of the special features of the Darwinian conception of nature. That there are carnivorous animals which take heavy toll in every generation of the progeny of the animals on which they prey, and that there are herbivores which decimate the plants in every generation had long been known, but it is only since Darwin's time that sufficient attention has been paid to the facts that, in addition to this regular destruction, there exists between the members of a species a keen competition for space and food, which limits multiplication, and that numerous individuals of each species perish because of unfavourable climatic conditions. The "struggle for existence," which Darwin regarded as taking the place of the human breeder in free nature, is not a direct struggle between carnivores and their prey, but is the assumed competition for survival between individuals _of the same_ species, of which, on an average, only those survive to reproduce which have the greatest power of resistance, while the others, less favourably constituted, perish early. This struggle is so keen, that, within a limited area, where the conditions of life have long remained unchanged, of every species, whatever be the degree of fertility, only two, _on an average_, of the descendants of each pair survive; the others succumb either to enemies, or to disadvantages of climate, or to accident. A high degree of fertility is thus not an indication of the special success of a species, but of the numerous dangers that have attended its evolution. Of the six young brought forth by a pair of elephants in the course of their lives only two survive in a given area; similarly, of the millions of eggs which two thread-worms leave behind them only two survive. It is thus possible to estimate the dangers which threaten a species by its ratio of elimination, or, since this cannot be done directly, by its fertility.

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