forms, remove the arsenic acid which may be present with acid. 7. Test another portion for FLUORINE (§ 147, 7). 8. If the solution is yellow, CHROMIC ACID is present. To remove all doubt on the point, acidify a portion of the solution with acetic acid, and test with acetate of lead. ε. Acidify the remainder of the solution with hydrochloric 210 acid, evaporate to dryness, and treat the residue with hydrochloric acid and water. If a residue is left which refuses to dissolve even in boiling water, this consists of SILICIC ACID. Test the hydrochloric acid solution now in the usual way for those bases which, being soluble in caustic alkalies, may be present. ε b. Dissolve the residue left in 4 (208) in hydrochloric acid 211 (effervescence indicates the presence of alkaline earths), and test the solution for the bases as directed in § 188. (If much silicic acid has been found in & (210), it is advisable to evaporate the solution of the residue to dryness, and to treat the residuary mass with hydrochloric acid and water, in order that the silicic acid remaining may also be removed as completely as possible.) 5. If you have found in 4 that the residue insoluble in acids 212 contains a silicate, treat a separate portion of it according to the directions of § 205, 2 (228), to ascertain whether or not this silicate contains alkalies. 6. If a residue is still left undissolved upon treating the 213 residue left in 4 with hydrochloric acid (211), this may consist either of silicic acid, which has separated, or of an undecomposed portion of sulphate of baryta; it may, however, also be fluoride of calcium, and if it is dark-colored, chrome-ironstone, as the lastnamed two compounds are only with difficulty decomposed by the method given in 4. I would therefore remind the student that fluoride of calcium may be readily decomposed by means of sulphuric acid; and, as regards the decomposition of chrome-ironstone, I can recommend the following method, first proposed by Hart: Project the fine powder into 8 times the quantity of fused borax, stir the mixture frequently, and keep the crucible for halfan-hour at a bright red heat. Add now to the fusing mass carbonate of soda so long as effervescence continues, and then finally add 3 times the weight of the chrome-ironstone of a mixture of equal parts of carbonate of soda and nitrate of potassa, whilst actively stirring the mixture with a platinum wire. Let the mass cool, and, when cold, boil it with water. 7. If the residue insoluble in acids contained silver, you have 214 still to ascertain whether that metal was present in the original substance as chloride, bromide, iodide, &c., of silver, or whether it has been converted into the form of chloride of silver by the treatment employed to effect the solution of the original substance. For that purpose, treat a portion of the original substance with boiling water until the soluble part is completely removed; then treat the residuary portion in the same way with dilute nitric acid, wash the undissolved residue with water, and test a small 226 ANALYSIS OF INSOLUBLE CYANIDES, FERROCYANIDES, ETC. [§ 202. sample of it for silver according to the directions of $201, d (203). If silver is present, proceed to ascertain the salt-radical with which the metal is combined; this may easily be effected by boiling the remainder of the residue in the first place with rather dilute solution of soda, filtering, and testing the filtrate, after acidifying it, for ferro- and ferricyanogen. Digest the washed residue now with finely granulated zinc and water, with addition of some sulphuric acid, and filter after the lapse of ten minutes. You may now at once test the filtrate for chlorine, bromine, iodine, and cyanogen; or you may first throw down the zinc with carbonate of soda, in order to obtain the salt-radicals in combination with sodium. SECTION II. PRACTICAL COURSE IN PARTICULAR CASES. I. SPECIAL METHOD OF EFFECTING THE ANALYSIS OF CYANIDES, $ 202. THE analysis of ferrocyanides, ferricyanides, &c., by the common 215 method is often attended by the manifestation of such anomalous reactions as easily to mislead the analyst. Moreover, acids often fail to effect their complete solution. For these reasons it is advisable to analyze them, and mixtures containing such compounds, by the following special method: Treat the substance with water until the soluble parts are entirely removed, and boil the residue with strong solution of potassa or soda; after a few minutes ebullition add some carbonate of soda, and boil again for some time; filter, should a residue remain, and wash the latter. 1. The residue, if any has been left, is now free from cya- 216 nogen, unless the substance under examination contains cyanide of silver, in which case the residue would of course still contain cyanogen. Examine the residue now by the common method, beginning at § 178, 2 (34). 2. The solution or filtrate, which, if combinations of com- 217 pound cyanogen radicals were originally present, contains these combined with alkali metals, may also contain other acids, which have been separated from their bases by the process of boiling with carbonate of soda, and lastly also, such oxides as are soluble in caustic alkalies. * Before entering upon this course of analysis, consult the special remarks to the paragraph (§ 202), page 274, in the Third Section. Treat the solution as follows: a. Mix the alkaline fluid with a sufficient quantity of hydro- 218 sulphuric acid. a. No permanent precipitate is formed. Absence of zinc and lead. Pass on to b. B. A permanent precipitate is formed. Add to the fluid. a little yellow sulphide of sodium, heat moderately, filter, wash the precipitate, and treat the filtrate as directed in b. Dissolve the washed precipitate in nitric acid, and examine the solution for copper, lead, zinc, and other metals of the fourth group, which may, in the same way as copper, have passed into the alkaline solution, by the agency of organic matters. b. Mix the alkaline fluid, which contains now also sulphide 219 of an alkali metal, with nitric acid to acid reaction, and, if necessary, add once more hydrosulphuric acid. Absence of mercury and of a. No precipitate is formed. c. The fluid, acidified with nitric acid, and therefore abun- 220 Evaporate ẞ to dryness, and heat the residue to fusion. Pour the fused mass upon a piece of porcelain, boil with water, filter, and examine the residue for IRON, MANGANESE, COBALT, and ALUMINA. Test a portion of the filtrate (if yellow) for CHROMIC ACID, the remainder for ALUMINA—which may have passed partially or completely into the solution, through the agency of the caustic alkalies formed, in the process of fusion, from the nitrates of the alkalies present. II. ANALYSIS OF SILICATES. Whether the body to be analyzed is a silicate, or contains one, is 221 ascertained by the preliminary examination with phosphate of soda and ammonia before the blowpipe; since, in the process of fusion, the metallic oxides dissolve, whilst the separated silicic acid floats about in the liquid bead as a transparent, swollen mass. The analysis of the silicates differs, strictly speaking, from the common course only in so far as the preliminary treatment is concerned, which is required to effect the separation of the silicic acid from the bases, and to obtain the latter in solution. The silicates and double silicates are divided into two distinct classes, which require respectively a different method of analysis; viz., (1) silicates decomposable by acids (hydrochloric acid, nitric acid, sulphuric acid) and (2) silicates which are not decomposed by acids. To ascertain to which of these two classes a silicate belongs, reduce it to a very fine powder, and digest a portion with hydrochloric acid at a temperature near the boiling point. If this fails to decompose it, try another portion with tolerably concentrated sulphuric acid, and apply heat. If this also fails, after some time, to produce the desired effect, the silicate belongs to the second class. Whether decomposition has been effected by the acid or not may generally be learned from external indications, as a colored solution forms almost invariably, and the separated gelatinous, flocculent, or finely-pulverulent hydrate of silicic acid takes the place of the original heavy powder which grated under the glass rod with which it was stirred; the separated hydrate of silicic acid will, after filtering and washing, dissolve in boiling solution of potassa or soda, which affords an additional means of identification. Before proceeding further, test a portion of the pulverized compound also for water, by heating it in a perfectly dry glass tube. If the substance contains hygroscopic moisture it must first be dried by protracted exposure to a temperature of 212° F. Apply a gentle heat at first, but ultimately an intense heat, by means of the blowpipe; you may also conveniently combine with this a preliminary examination for fluorine (§ 147, 8). A. SILICATES DECOMPOSABLE BY ACIDS. $204. a. Silicates decomposable by hydrochloric acid or by nitric acid.* 1. Digest the finely pulverized silicate with hydrochloric acid at 222 a temperature near the boiling point, until complete decomposition is effected, filter off a small portion of the fluid, evaporate the remainder, together with the silicic acid suspended therein, to dryness, and expose the residue to a temperature somewhat exceeding 212° F., with constant stirring, until no more, or very few, hydrochloric acid fumes escape; allow it to cool, moisten the residue with hydrochloric acid, or as the case may be, with nitric acid, afterwards add a little water, and heat gently for some time. This operation effects the separation of the silicic acid, and the solution of the bases in the form of chlorides. Filter, wash the residue thoroughly, and examine the solution by the common method, beginning at § 188.+ To be quite safe, the residuary silicic acid may be digested with ammonia, filtered, and the filtrate tested for silver, by supersaturation with nitric acid. 2. As in silicates, and more particularly in those decomposed by 223 *Nitric acid is preferable to hydrochloric acid in cases where compounds of silver or lead are present. + Minute traces of titanic acid are occasionally met with in silicates. The titanic acid present passes into the hydrochloric acid solution, if the separation of the silicic acid has been effected on the water-bath, and is subsequently thrown down with the bases precipitable by ammonia (sesquioxide of iron and alumina). Upon iguiting the precipitate, and then treating it with concentrated hydrochloric acid, the greater portion of the titanic acid remains undissolved. On filtering it, the washings generally look milky when passing through the filter. It is more closely examined before the blowpipe; compare § 103 (H. Rose). hydrochloric acid, there are often found other acids, as well as metalloids, the following observations and instructions must be attended to, that none of these substances may be overlooked :— a. SULPHIDES of METALS and CARBONATES are detected in the process of treating with hydrochloric acid. B. If the separated silicic acid is black, and turns subsequently white upon ignition in the air, this indicates the presence of CARBON or of ORGANIC SUBSTANCES. In presence of the latter, the silicates emit an empyreumatic odor upon being heated in a glass tube. 7. Test the portion of the hydrochloric acid solution filtered off before evaporating (222), for PHOSPHORIC ACID (ARSENIC ACID, which may happen to be present), and SULPHURIC ACID, by means of molybdic acid, and chloride of barium. d. BORACIC ACID is best detected by fusing a portion 224 of the substance in a platinum spoon with carbonate of soda and potassa, boiling the fused mass with water, and examining the solution for boracic acid by the method given in § 145, 6. ε. With many silicates, boiling with water is sufficient to dissolve the metallic CHLORIDES present, which may then be readily detected in the filtrate by means of solution of nitrate of silver; the safest way, however, is to dissolve the mineral in dilute nitric acid, and test the solution with nitrate of silver. <. Metallic FLUORIDES, which often occur in silicates in greater or smaller proportion, are detected by the methods described § 147. b. Silicates which resist the action of hydrochloric acid, but are decomposed by concentrated sulphuric acid. Heat the finely pulverized mineral with moderately concentrated 225 pure sulphuric acid (best in a platinum dish), finally drive off the sulphuric acid, boil the residue with hydrochloric acid, dilute, filter, and treat the filtrate as directed § 188; and the residue, which, besides the separated silicic acid, may contain also sulphates of the alkaline earths, &c., according to the directions of § 201. If you wish to examine silicates of this class for acids and salt-radicals, treat a separate portion of the substance according to the directions of § 205. B. SILICATES WHICH ARE NOT DECOMPOSED BY ACIDS. § 205. As the silicates of this class are most conveniently decomposed by 226 fusion with carbonate of soda and potassa, the portion so treated cannot, of course, be examined for alkalies. The analytical process is therefore properly divided into two principal parts, viz., a portion of the mineral is examined for the silicic acid and the bases, with the exception of the alkalies, whilst another portion is specially examined for the latter.-Besides these, there are some other experiments required, to obtain information as to the presence or absence of other acids. 1. Detection of the silicic acid and the bases, with the exception of the alkalies. Reduce the mineral to a very fine powder, mix this with 4 227 parts of carbonate of soda and potassa, and heat the mixture in a |