precipitated by it, and it acts upon those, because the metals have a strong attraction for that substance which the Prussian alkali contains; at the same time the acid acts upon the alkali, and produces a double exchange: and thus the metal is precipitated. If iron, it is precipitated in a Prussian blue. If the Prussian alkali is produced by calcination with blood, it will be very difficult to saturate it entirely; and if any part is not saturated, it will precipitate a calcareous earth. The only way of saturating it entirely is by digesting it in Prussian blue already prepared, which tinges it yellow. The metallic substances found in mineral waters are seldom any other than iron or copper, and we may at once discover their precipitates by their colour. When we desire to know more certainly if iron is contained in water, and also the quantity of it, we may use solution of galls, which are the most exquisite test of the minutest quantity of iron. Copper may be discovered by the addition of a volatile alkali, which produces a blue colour, by first precipitating the copper, and then re-dissolving it; though the Prussian alkali answers better. The presence of copper, too, may be discovered by throwing into the water a piece of very bright iron; very soon after (especially if the liquor is warm) the copper is deposited on the surface of the iron; but this seldom happens, unless there is a considerable quantity of copper; which is rarely the case in mineral waters. Zinc is precipitated white by the Prussian alkali; but as the precipitate from many other metals is of the same colour, we must have recourse to evaporation to dryness, and then examine the residuum. If it is zinc, it will convert a small plate of copper into brass. If arsenic is supposed to be contained in mineral waters, we must evaporate them to dryness, and judge by the smell when the residuum is made red-hot, or by trying if it tinges copper white. LECTURE XXXIV. CHEMISTRY. VEGETABLE AND ANIMAL SUBSTANCES. THE simple substances which enter into the composition of vegetable bodies are very few. As constituent matters, we may confine them to carbon, hydrogen, oxygen, and nitrogen; and the latter exists in very small quantities. In animal bodies the list may be increased, as phosphorus and lime enter in considerable quantities, at least into the composition of the solid parts, as the bones, &c. The alkalies also, and some of the metals, are found in animal and vegetable bodies, but the latter in too small proportions, and too casually dispersed, to allow us to regard them as constituent parts. From these few simple principles, however, a diversity of compounds are formed. It will conduce much to perspicuity to treat separately of vegetable and animal substances. The physiology of both is foreign to the object of these lectures. Chemistry is concerned with them only when they have ceased to live. It treats of the substances of which they are composed, and of the changes which these substances undergo. The following are all the substances which have hitherto been found to exist in vegetables. 11. Extractive principle. 16. Wax. 17. Resins. 20. Sandarach. 23. Suher, or Cork. 25. Earths. 26. Metals. "The three last," Dr. Thomson judiciously remarks," are scarcely entitled to the name of vegetable principles. It is highly probable," he adds, "that they are taken up ready formed, and deposited without alteration in the vegetables which contain them, whereas the other twentythree genera consist of substances which owe their formation to the processes of vegetation." Of some of these, however, as the acids, oils, and resins, we have been under a necessity of treating in the preceding Lectures. I shall, therefore, not enlarge on them in this, but content myself with a reference to the lectures where they are to be found. 1. Sugar is a substance which is contained more or less in most vegetables. Some, how ever, as the sugar-cane and the sugar-maple of America, contain it in much larger quantities than others, so as to render the culture of these plants, and the preparation of the article from them, a matter of great commercial importance. Sugar is decomposed both by heat and mixture; and by the most accurate experiments it is found to be composed entirely of oxygen, carbon, and hydrogen. It is therefore a vegetable oxide. The proportions of these matters were found by Lavoisier to be 64 parts oxygen. 8 hydrogen. 100 These proportions must, however, vary considerably in the sugars produced from different plants, and they must frequently have united with them some heterogeneous matters. The beet, the carrot, the parsnip, the sap of many trees, and all the different kinds of grain, contain sugar in considerable abundance. According to the calculations of M. Achard of Berlin, twenty pounds of beet root will yield one of sugar, and a German square mile of land (sixteen square miles English) would produce white beet enough to furnish the Prussian dominions with sugar. The saccharine matter is so profusely diffused in the vegetable kingdom, that we see the bees collect it in large quantities from the flowers of |