convert these elements into compounds of known composition, to separate the new compounds formed from one another, to weigh these, and to calculate from the results the quantities of the separate elements. Organic analysis, therefore, is based upon the same principle upon which rest most of the methods of separating and determining inorganic compounds.

The decomposition of most organic substances into distinctly characterized and readily separable products, which will permit an accurate determination of weight, offers no great difficulties, and organic analysis is therefore usually one of the more easy tasks of analytical chemistry;and as, from the limited number of the elements which constitute organic bodies, there is necessarily a great sameness in the products of their decomposition, the analytical process is always very similar, and a few methods suffice for all cases. It is principally ascribable to this latter circumstance that organic analysis has so speedily attained its present high degree of perfection :-the constant examination and improvement of a few methods by a great number of chemists could not fail to produce this result.

Organic analysis may have for its object either simply to ascertain the relative proportion of the constituent elements of a substance,-thus, for instance, woods are analysed to ascertain their value as fuel, fats to ascertain their capacity of furnishing light-or to determine not only the relative proportions of the constituent elementary atoms, but also their absolute quantity, that is, to determine the exact number of equivalents of carbon, hydrogen, oxygen, &c. &c., which constitute 1 equivalent of the analysed compound. In scientific investigations we have invariably the latter object in view, although not yet able to achieve it in all cases. These two distinct objects cannot well be attained by one and the same operation; each requires a different and distinct process.

The methods by which we ascertain the relative proportions of the constituent elements of organic compounds, may be called collectively, "elementary or ultimate analysis of organic bodies," in a more restricted sense; whilst the methods which reveal to us the absolute number of elementary equivalents constituting the complex equivalent of the analysed compound, may be styled "determination of the equivalents of organic bodies."

The success of an organic analysis depends upon two conditions; viz., 1, upon the selection of the proper method; and, 2, upon the correct performance of the necessary operations and processes: expertness in this branch may be readily acquired by any one endowed with some patience, clearness of perception, and skill in chemical manipulation. The selection of the method depends upon the knowledge of the constituents of the compound under examination, and the method selected will always require certain modifications, according to the different properties and state of aggregation of these constituents. Before we can proceed, therefore, to describe the various methods applicable in the different cases that may occur, we have first to occupy ourselves here with the general means of ascertaining the constituent elements of organic bodies.

I. QUALITATIVE EXAMINATION OF ORGANIC BODIES.

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It is not necessary for the correct selection of the proper method, to

know all and every one of the elements of an organic compound, since the presence or absence of some of them-of oxygen, for instance-has not the slightest modifying influence upon the manner of proceeding with the analysis. But with regard to other elements, such as nitrogen, sulphur, phosphorus, chlorine, iodine, bromine, &c. &c., and also the various metals, it is absolutely indispensable that the operator should know positively whether either of them is present. This may be ascertained in the following manner :

1. Testing for Nitrogen.

Substances containing a tolerably large amount of nitrogen exhale upon combustion, or when intensely heated, the well-known smell of singed hair or feathers. No further test is required if this smell is distinctly perceptible; otherwise the following experiments are resorted

to:

a. The substance under examination is mixed with hydrate of potassa in powder, or with soda-lime (§ 66, 5), and the mixture heated in a testtube. If the examined substance contains nitrogen, ammonia will be evolved, which may be readily detected by its peculiar odor, by its reaction upon vegetable colors, and by the formation of white fumes when brought into contact with volatile acids. Should these reactions fail to afford positive proof of the presence of nitrogen, every doubt may be removed by the following experiment:-Heat a somewhat larger portion of the substance, in a short tube, with an excess of soda-lime, and conduct the products of the combustion into dilute hydrochloric acid; evaporate the acid on the water-bath, dissolve the residue in a little water, and mix the solution with bichloride of platinum and alcohol. Should no precipitate form, even after the lapse of some time, the examined substance may be considered free from nitrogen.

b. Lassaigne has recently proposed another method, which is based upon the property of potassium to form cyanide of potassium when ignited with a nitrogenous organic substance. The following is the best mode of performing the experiment:

Heat the substance under examination, in a test-tube, with a small lump of potassium, and after the complete combustion of the whole of the potassium, treat the residue with a little water (cautiously); filter the solution, add 2 drops of solution of sulphate of protoxide of iron containing some sesquioxide, digest the mixture a short time, and add hydrochloric acid in excess. The formation of a blue or bluish-green precipitate or coloration proves the presence of nitrogen.

Both methods are delicate; a is the one most generally employed; it fully answers the purpose in nearly all cases.

c. In organic substances containing oxides of nitrogen, the presence of nitrogen cannot be shown by either a or b, but it may be readily detected by heating the substance in a tube, when the evolution of red acid fumes, imparting a blue tint to iodide of starch paper, will incontestably prove it.

2. Testing for Sulphur.

a. Solid substances are fused with about 12 parts of pure hydrate of potassa, and 6 parts of nitrate of potassa. Or they are intimately mixed with some pure nitrate of potassa and carbonate of soda; nitrate of potassa is then heated to fusion in a porcelain crucible, and the mixture

gradually added to the fusing mass. The mass is allowed to cool, then dissolved in water, and the solution tested with baryta, after previously acidifying with hydrochloric acid.

b. Fluids are treated with fuming nitric acid, or with a mixture of nitric acid and chlorate of potassa, at first in the cold, finally with application of heat; the solution is tested as in a.

c. As the methods a and b serve simply to indicate the presence of sulphur in a general way, but afford no information regarding the state or form in which that element may be present, I add here another method, which serves to detect only the sulphur in the non-oxidized state in organic compounds.

Boil the substance with strong solution of potassa, and evaporate nearly to dryness. Dissolve the residue in a little water, pour the solution into the flask A (Fig. 81), and slowly add dilute sulphuric acid through the funnel-tube c; if sulphur is present, the slip of paper b, which has been thoroughly moistened with solution of acetate of lead, and then touched with a few drops of solution of carbonate of ammonia, will turn brown. I need hardly mention that the cork must not fit air-tight into the mouth of A. Instead of in the manner described, the sulphide of potassium formed may be detected also by means of nitroprusside of sodium, or by just acidifying the dilute solution with hydrochloric acid, and adding a few drops of a mixture of sesquichloride of iron, and ferricyanide of potassium (See "Qual. Analysis,” § 156).

3. Testing for Phosphorus.

Fig. 81.

The methods described in 2, a, and b, may likewise serve for phosphorus. The solutions obtained are tested for phosphoric acid with sulphate of magnesia; or with sesquichloride of iron, with addition of acetate of soda; or with molybdate of ammonia (compare "Qualitative Analysis"). In method b, the greater part of the excess of nitric acid must first be removed by evaporation.

4. Testing for Inorganic Substances.

A portion of the substance under examination is heated on platinum foil, to see whether or not a residue remains. When acting upon difficultly combustible substances, the process may be accelerated by heating the spot which the substance under examination occupies on the platinum foil, to the most intense redness, directing the flame of the blowpipe upon the corresponding point of the lower surface of the foil. The residue is then examined by the usual methods. That volatile metals in volatile organic compounds-e.g., arsenic in kakodyl-cannot be detected by this method, need hardly be mentioned.

These preliminary experiments should never be omitted, since neglect in this respect may give rise to very great errors. Thus, for instance, taurine, a substance in which a large proportion of sulphur was afterwards found to exist, had originally the formula C, N H, O,, assigned for its composition. The preliminary examination of organic substances for chlorine, bromine, and iodine, is generally unnecessary, as these elements do not occur in native organic compounds; and as their presence in

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compounds artificially produced by the action of the salt-radicals requires generally no further proof. Should it, however, be desirable to ascertain positively whether a substance does or does not contain chlorine, iodine, or bromine, this may be done by the same methods which we shall have occasion to describe in the quantitative determination of organic compounds.

II. QUANTITATIVE DETERMINATION OF THE ELEMENTS IN ORGANIC BODIES.

It is not my intention to give an account of the rise and progress of the science of organic analysis; I shall therefore confine myself to the description of the most simple, precise, and universally applicable methods, omitting all the rest. The more simple methods, which may be performed by way of practice, will be most fully described; the more complicated methods, which presuppose a more advanced knowledge of the general manipulations of organic analysis, will be given more briefly.

The accuracy of the results depends both upon the appropriate construction and arrangement of the apparatus required for the various analytical processes, and upon the proper execution of these processes. I am anxious, therefore, to impress upon the student the necessity of bestowing equal attention and diligence upon both; the rules which I have laid down are the fruits of long experience and of innumerable experiments.

A. ANALYSIS OF COMPOUNDS WHICH CONSIST SIMPLY OF CARBON AND HYDROGEN, OR OF CARBON, HYDROGEN, AND OXYGEN.

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The principle of the method which serves to effect the quantitative analysis of such compounds, and which owes its present perfection to Professor Liebig, is exceedingly simple. The substance under examination is burned, and thus converted into carbonic acid and water; these products are then separated from each other and weighed, and the carbon of the analysed substance is calculated from the weight of the carbonic acid, the hydrogen from that of the water. If the sum of the calculated weight of the carbon and hydrogen is equal to the original weight of the substance, the absence of oxygen is at once satisfactorily proved; should the sum, on the other hand, be less than the original weight of the substance, the difference will at once indicate the amount of oxygen originally present in the analysed compound.

The combustion is effected either by igniting the organic substance with oxygenized bodies which readily part with their oxygen (oxide of copper, chromate of lead, &c. &c.); or by means of free oxygen gas; or, finally, at the expense both of free and combined oxygen.

[NOTE. The only attempt at determining the oxygen in organic bodies in the direct way, has been made by Baumhauer ("Annal. d. Chem. u. Pharm.," 90, 228).

The following is the process given by this chemist :-Mix the organic substance with oxide of copper, in the usual way, and heat in a glass tube open at both ends. Collect the carbonic acid and the water in the usual way. The two ends of the apparatus are connected with accurately graduated glass-tubes, of which the one at the posterior end is filled with oxygen gas. At the termination of the process of combus

tion, this oxygen gas is passed over the ignited oxide of copper and reduced copper, by which means the reduced metal is reoxidized. Allow the apparatus to cool, read off the volume of gas in the two graduated tubes (as has been done also before the process), and then determine the weight of the carbonic acid and water. Make due correction for the state of the barometer and thermometer, then deduct the volume of gas found in the two tubes after the combustion, from the volume before the process; the difference gives the quantity of oxygen absorbed by the reduced copper. Deduct now this difference from the joint amount of oxygen in the carbonic acid and water formed; the difference gives the quantity of oxygen in the analysed substance. As the total amount of gas in the apparatus cannot be accurately known, the results of the operation can only be expected to be correct if nearly the same pressure and temperature are maintained at the end as at the beginning of the experiment; there must also be no alteration in the volume of the tube. Baumhauer analysed oxalic acid and oxalate of lead by this method, with the following most satisfactory results :—

0-9895 grm. of oxalic acid gave 0.969 of carbonic acid and 0.203 of water. The total volume of gas in the apparatus, duly corrected, was, before the process, 485.90 c.c., after the process 360 33 c.c. Calculating from this the per-centage composition of oxalic acid, we have

The analysis of oxalate of lead gave 16:30 per cent. of oxygen, instead of 16-26, as theoretically calculated.]

a. SOLID BODIES.*

a. Readily combustible, non-volatile substances (e.g., sugar, starch, tartaric acid, and the far greater number of solid organic bodies).

1. Liebig's Method.

I. APPARATUS AND PREPARATIONS REQUIRED FOR ORGANIC ANALYSIS.

§ 174.

The following is a complete list of everything requisite for the performance of an organic analysis :—

1. THE SUBSTANCE INTENDED FOR ANALYSIS. This must be most finely pulverized and perfectly pure and dry; for the method of drying organic substances, I refer to § 26.

2. A TUBE IN WHICH TO WEIGH THE SUBSTANCE.-A small perfectly dry glass tube, about 4 or 5 centimetres long, and about 1 centimetre wide, is used for this purpose; the weight of tube must be accurately determined to within a centigramme. It is advisable to place the tube in the drying

Fig. 82.

For the analysis of fats, waxy bodies, &c., which cannot be reduced to powder, I refer to § 182.