PART II. PRELIMINARY REMARKS ON THE COURSE OF QUALITATIVE ANALYSIS IN GENERAL AND ON THE PLAN OF THIS PART OF THE PRESENT WORK IN PARTICULAR. THE knowledge of reagents and of the deportment of other bodies with them enables us to ascertain at once whether a compound of which the physical properties permit an inference as to its nature, is in reality what we suspect it to be. Thus, for instance, a few simple reactions suffice to show whether a body which appears to be calcareous spar, is really carbonate of lime, and that another, which we hold to be gypsum, is actually sulphate of lime. This knowledge usually suffices also to ascertain whether a certain body is present or not in a compound; for instance, whether or not a white powder contains subchloride of mercury. But if our design is to ascertain the chemical nature of a substance entirely unknown to us if we wish to discover all the constituents of a mixture or chemical compound-if we intend to prove that, besides certain bodies which we have detected in a mixture or compound, no other substance can possibly be present-if consequently a complete qualitative analysis is our object, the mere knowledge of the reagents, and of the reactions of other bodies with them, will not suffice for the attainment of this end; this requires the additional knowledge of a systematic and progressive course of analysis, in other words, the knowledge of the order and succession in which solvents, and general and special reagents, should be applied, both to effect the speedy and certain detection of every component element of a compound or mixture, and to prove with certainty the absence of all other substances. If we do not possess the knowledge of this systematic course, or if, in the hope of attaining our object more rapidly, we adhere to no method whatever in our investigations and experiments, analyzing becomes (at least in the hands of a novice) mere guess-work, and the results obtained are no longer the fruits of scientific calculation, but mere matters of accident, which sometimes may prove lucky hits, and at others total failures. Every analytical investigation must therefore be based upon a definite method. But it is not by any means necessary that this method should be the same in all cases. Practice, reflection, and a due attention to circumstances will, on the contrary, generally lead to the adoption of different methods for different cases. However, all analytical methods agree in this, that the substances present or supposed to be present in a compound or mixture, are in the first place classed into certain groups, which are then again subdivided, until the individual detection of the various substances present is finally accomplished. The diversity of analytical methods depends partly on the order and succession in which reagents are applied, and partly on their selection. Before we can venture upon inventing methods of our own for individual cases, we must first make ourselves thoroughly conversant with a certain definite course, or system, of chemical analysis in general. This system must have passed through the ordeal of experience, and must be adapted to every imaginable case, so that afterwards, when we have acquired some practice in analysis, we may be able to determine which modification of the general method will in certain given cases most readily and rapidly lead to the attainment of the object in view. The exposition of such a systematic course, adapted to all cases, tested by experience, and combining simplicity with the greatest possible security, is the object of the First Section of the second part of this work. The elements and compounds comprised in it are the same which we have studied in Part I., with the exception of those given in that part simply by way of appendix, or printed in smaller type. The First Section of the Second Part consists of PRACTICAL INSTRUCTIONS IN ANALYSIS, wherein I have laid down a systematic course which, with due care and attention, will, by progressive steps, lead speedily and safely to the attainment of the end in view. The subdivisions of this practical course are: 1, Preliminary examination; 2, Solution; 3, Actual analysis. The third subdivision (the actual analysis) is again subdi vided into, (1) Examination of compounds in which but one base and one acid are assumed to be present; and, (2) Examination of mixtures or compounds in which all the substances treated of in the present work are assumed to be present. With respect to the latter section, I have to remark that where the preliminary exami nation has not clearly demonstrated the absence of certain groups of substances, the student cannot safely disregard any of the paragraphs to which reference is made in consequence of the reactions observed. In cases where the intention is simply to test a compound or mixture for certain substances, and not to ascertain all its constituents, it will be easy to select the particular numbers which ought to be attended to. As the construction of a universally applicable systematic course of analysis requires due regard to, and provision for, every contingency that may possibly arise, it is self-evident that, though in the system here laid down the various bodies comprised in it have been assumed to be mixed up together in every conceivable way, it was absolutely indispensable to proceed throughout upon the supposi tion that no foreign organic matters whatever were present, since the presence of such matters would of course tend to prevent or obscure many reactions, and variously modify others. Although the general analytical course laid down here is devised and arranged in a manner to suit all possible contingencies, with a very few exceptions, still there are special cases in which it may be advisable to modify it. A preliminary treatment of the substance is also sometimes necessary, before the actual analysis can be proceeded with; the presence of coloring, slimy, organic matters more especially requires certain preliminary operations. Not to leave the student without a guide in these special cases, the Second Section of this Part will be found to contain a detailed description of the methods employed to effect the analysis of a few important compounds and mixtures which chemists are frequently called upon to examine. Some of these methods show how the analytical process becomes simplified as the number of substances decreases to which regard must be had in the analysis. In conclusion, as an intelligent and successful pursuit of analysis is possible only with an accurate knowledge of the principles whereon the detection and separation of bodies depend, since this knowledge alone can furnish the student with a guide to the selection of the proper reagents, and the order in which they ought to be applied, I have given in the Third Section of the Second Part an explanation and elucidation of the general analytical process, with numerous additions to the practical operations. As this third section may properly be regarded as the key to the first and second sections, I strongly recommend students to make themselves early and thoroughly acquainted with it. I have devoted a special section to this theoretical explanation of the process, as I think it will be understood better in a connected form, than it would have been by explanatory additions to the several paragraphs, which, more over might have materially interfered with the plainness and perspicuity of the plan of the practical process. In this Third Section I have also indicated in what residues, solutions, precipitates, &c., as obtained in the systematic course, the operator may look for the elements of more rare occurrence, and how these bodies may be systematically and surely detected in analyses where they are likely to be present. SECTION I. PRACTICAL PROCESS FOR THE ANALYSIS OF COM fru Sex Page 370 I. PRELIMINARY EXAMINATION.* $178. 1. EXAMINE, in the first place, the physical properties- 1 color, shape, hardness, gravity, odor, &c.-of the substance intended for analysis, since these will often enable you in some measure to infer its nature. Before proceeding to the application of any chemical process, you must always consider how much of the substance to be analyzed you have at command, since it is necessary, at this early period of the examination, to calculate the quantity which may safely be used in the preliminary investigation. A reasonable economy is in all cases advisable, even though you may possess the substance in large quantities; but, under all circumstances, let it be a fixed rule, never to use at once the whole of what you possess of a substance, but always to keep a portion of it for unforeseen contingencies, and for confirmatory experiments. A. THE BODY UNDER EXAMINATION IS SOLID. § 179. 1. The substance is fit for examination if in powder or in 2 minute crystals; but if in larger crystals or in solid pieces, it is necessary in the first place to reduce a portion of it to fine powder, if practicable. Bodies of the softer kind may be triturated in a porcelain mortar; those of a harder nature must first be broken into small pieces in a steel mortar, or upon a steel anvil, and the pieces then be triturated in an agate mortar. 2. PUT SOME OF THE POWDER INTO A GLASS TUBE, SEALED 3 AT ONE END, ABOUT 2 INCHES LONG AND THS OF AN INCH WIDE, AND HEAT, first gently over the spirit or gas-lamp, then intensely in the blowpipe flame. The reactions resulting may * Consult also the observations and additions in the Third Section. |