In order to bring the tests for analogous 3odies together, I shall here interrupt the course .f the analysis to describe the methods of pro eeding with those substances for which occa ionally, though more rarely, we have to look. Iodine, in organic fluids, always occurs in le form of an iodide, and is not met with in le human body in its normal condition. We mst evaporate to dryness and treat the residue ith akohol. The iodide will be dissolved ; e again evaporate to dryness and re-dissolve .water : (if the quantity be not very minute, 3s preliminary process may be dispensed ith, merely concentrating the liquid and lowing it to cool ;) iodine may now be ttected by adding a little cold solution of trch, and pouring into the mixture a few .,ops of solution either of chlorine or of chlo ride of lime (bleaching liquor), when a blue colour, more or less intense, is produced. The quantitative estimation of iodine in these ana lyses is seldom required ; when it is, a neutral solution of chloride of palladium is added to the solution, accurately neutralized, and the whole set aside in a warm place for twenty four hours, a black precipitate of iodide of pal ladium forms : it should be collected on a vveighed filter and dried at a very gentle heat, otherwise part of the iodine escapes. 100 grs. of iodide of palladiurn contain 70 of iodine. By suspending this iodide in water and adding starch and a little chlorine water, the blue colour is produced as usual.
Fluorine, when present, and it appears to be a universal constituent of bones, is always in exceedingly minute quantity. To discover it we incinerate the dried 'natter, pulverize and make it into a thin cream with oil of vitriol in a shallow platinum crucible ; instead of its usual cover the mouth is closed by a piece of flat glass, the under surface of which has been covered with a film of melted bees' wax or some resinous varnish; when firm or dry, a few characters are traced with a sharp point to expose the glass underneath ; the glass is pressed upon the crucible so as completely to close it, and the whole heated over a spirit lamp for a quarter of an hour. The glass is kept cool by a piece of moistened paper. If any fluorine be present, the traces upon the glass from which the wax has been removed will be more or less corroded ; the superfluous wax may be removed by oil of turpentine, and the corrosion may be rendered distinct by rubbing a little powdered charcoal over the surface. If any marks are produced, it is an unequivocal proof of the presence of fluorine. This method, however, is not very delicate.
Free sulphur is detected by boiling the sub stance with solution of potash ; if this element be present in the unoxidized state, a black pre cipitate of sulphuret of lead is formed on add ing a few drops of acetate of lead.
If we desire to know the quantity of free sulphur, we first satisfy ourselves of the ab sence of sulphuric acid, Or determine its quan tity accurately by the method already de scribed ; then deflagrate the substance or dry residue with eight parts of pure nitre and two of pure carbonate of potash, throwing the mixture in successive small portions into a platinum crucible heated to redness; the sulphur is thus converted into sulphuric acid at the expense of the oxygen of the nitre, and its quantity may be determined by dissolving the saline residue in water, supersaturating with nitric acid, and precipitating by a salt of baryta as usual : the process is one requiring more than ordinary care to ensure accuracy.
To resume the usual process of analysis, we now proceed to determine the bases. Most of the acids may be determined with consi -derable exactness before the organic matter has been destroyed by ignition; it is not so with the bases. Incineration should always precede an attempt to esthnate them. The second por tion of saline matter is dissolved in water as before, separating it thus into a solution (A), and an insoluble residue (B).
(A.) Polash.—The solution rarely contains any but the alkaline salts. lf, however, any of the earths are present, they must first be separated in the form of carbonates, by adding a mix ture of carbonate and caustic ammonia ; the filtered liquid is evaporated to dryness, the residue ignited to expel ammoniacal com pounds and re-dissolved in water, which then contains only salts of potash and soda. If sulphuric or phosphoric acids be present, it is necessary for a quantitative determination of each alkali to convert the mixed sulphates or phosphates into chlorides. The method for ,accomplishing this object is rather circuitous : chloride of barium in slight excess is added to the solution, which is filtered from the sulphate or phosphate of baryta that then precipitates ; the filtered liquid is heated with a mixture of caustic and carbonate of ammonia, again fil tered, evaporated to dryness, and ignited ; the bases are thus obtained in their desired con dition of chlorides; they are then carefully weighed, re-dissolved in a small quantity of water; bichloride of platinum in solution is added, and the whole evaporated to dryness on a water-bath. The dry residue is digested with rectified spirit, and the washing continued as long as the liquid passes coloured through the filter; the precipitate, consisting of anhydrous double chloride of platinum and potassium, is dried and weighed; 100 grs. indicate 19.43 of potash.