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.

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 examinations; 2, Solution; 3, Actual examination.

The third subdivision (the actual examination) is again subdivided 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 examination 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 supposition 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.

The Second Section of this Part will be found to contain a detailed description of the special 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 processes become 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 several paragraphs, which, moreover, might have materially interfered with the plainness and perspicuity of the plan of the practical process.

SECTION I.

PRACTICAL PROCESS FOR THE ANALYSIS OF COMPOUNDS AND MIXTURES IN GENERAL.

I. PRELIMINARY EXAMINATION.*

$173.

1. EXAMINE, in the first place, the physical properties-color, 1+ 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.

* Consult also the observations and additions in the Third Section.
+ These marginal numbers are simply intended to facilitate reference.

A. THE BODY UNDER EXAMINATION IS SOLID.

I. IT IS NEITHER A PURE METAL NOR AN ALLOY.

§ 174.

1. The substance is fit for examination if in powder or in minute 2 crystals; but if in larger crystals or in solid pieces, it is necessary in the first place to reduce a portion of it to 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 at one end, 3 about six centimètres long and five millimètres wide, and heat first gently over the spirit or gas-lamp, then intensely in the blowpipe flame. The reactions resulting may lead to many positive or probable conclusions regarding the nature of the substance. The following are the most important of these reactions, to which particular attention ought to be paid; it often occurs that several of them are observed in the case of one and the same substance.

a. THE SUBSTANCE REMAINS UNALTERED: absence of organic 4 matters, salts containing water of crystallization, readily fusible matters, and volatile bodies.

b. THE SUBSTANCE DOES NOT FUSE AT A MODERATE HEAT, BUT 5 SIMPLY CHANGES COLOR. From white to yellow, turning white again on cooling, indicates OXIDE OF ZINC; from white to yellowishbrown, turning to a dirty light yellow on cooling, indicates BINOXIDE OF TIN; if the color changes from white to brownishred, turning to yellow on cooling, and the body is fusible at a red heat, this indicates the presence of OXIDE OF LEAD; if the color changes from white to orange-yellow, or a deeper and more reddish tint, up to reddish-brown, turning pale yellow on cooling, and the body fuses at an intense red heat, this indicates the presence of TEROXIDE OF BISMUTH; if the color changes from red to black, turning red again on cooling, this indicates the presence of SESQUIOXIDE OF IRON, &c.

c. THE SUBSTANCE FUSES WITHOUT EXPULSION OF AQUEOUS 6 VAPOR. If on intense heating, gas (oxygen) is evolved, and a small fragment of charcoal thrown in is energetically consumed, NITRATES OF CHLORATES may be assumed to be present.

d. AQUEOUS VAPORS ARE EXPELLED, WHICH CONDENSE IN THE 7 COLDER PART OF THE TUBE: this indicates the presence either (a) of SUBSTANCES CONTAINING WATER OF CRYSTALLIZATION, in which case they will generally readily fuse, and re-solidify after expulsion of the water; many of these swell considerably whilst yielding up their water, e. g. (borax, alum); or (6) of decomposable HYDRATES, in which case the bodies often will not fuse; or (7) of anhydrous salts, holding water mechanically enclosed between their lamella-in which case the bodies will decrepitate; or (d) of bodies with moisture externally adhering to them.