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4 DIFFERENCE BETWEEN ORGANIC AND INORGANIC COMPOUNDS.

activity of the growing plant or animal as that of carbon, of hydrogen, or of oxygen; for no organized tissue has ever been found free from some of these saline substances. Besides these natural components of organic products, the chemist often artificially introduces other elements for the purpose of dissecting, as it were, these compounds, and of ascertaining the probable rational composition of the body under experiment. With this view he either produces new compounds which contain sulphur, phosphorus, chlorine, bromine, iodine, peroxide of nitrogen, or sulphuric acid; or he obtains others in which arsenic, antimony, zinc, and a variety of metallic bodies, are introduced into the original substance.

(1037) Organic and Inorganic Compounds. Many distinguished philosophers have attempted to draw the line which separates organic from inorganic chemistry. Laurent, for instance, has termed organic chemistry 'the chemistry of carbon,' and Liebig has defined it as the chemistry of compound radicles.' Few persons, however, would be disposed with Laurent, to consider carbonic anhydride as an organic compound, and many chemists regard sulphurous anhydride, which is undoubtedly a compound inorganic substance, as the radicle of a somewhat numerous series of bodies of inorganic origin.

There is, in fact, no definite line of demarcation between inorganic and organic products. Amongst the productions of organized nature, acids, alkalies, salts, and other bodies are met with, similar in chemical functions to those derived from inanimate nature; and all organic compounds, when once formed, are subject to precisely the same chemical laws as those which regulate the combinations and decomposition of bodies confessedly inorganic; but the composition of the former being generally much more complicated than that of the latter, the balance of chemical attractions in organic bodies is disturbed by slighter causes; and there are, consequently, an unnumbered variety of products generated by slight modifications of the various forces to which organic substances are subjected.

Still, for convenience sake, it is advisable to classify chemical compounds in some measure according to their origin; since those derived from the inorganic world, from the greater simplicity of their composition, afford to the student the most favourable instances for examining the fundamental laws of chemical combination, before he proceeds to the investigation of the more complicated products obtained from operations of the living plant or animal.

FREQUENCY OF ISOMERISM IN ORGANIC COMPOUNDS.

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The number of elementary atoms which enter into the formation of an inorganic compound rarely exceeds ten or twelve; whereas in bodies of organic origin they may have any degree of complexity, from the simplest known organic compound, hydrocyanic acid, which contains only three elements (HEN), to the complex substance stearin (57H1106) with its 173 atoms, or the still more complex compound albumen, the molecule of which (72H112N1822) contains not less than 225 atoms, and perhaps even more.

(1038) Frequency of Isomerism.-The number of organic compounds known to chemists is very great, and the list is perpetually undergoing increase; it therefore cannot excite surprise, if amongst them numerous instances of isomerism, metamerism, and polymerism should occur.

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The formation of isomerides, metamerides, and polymerides, as bodies which possess the same percentage composition may be termed, can only be accounted for by supposing that differences of chemical arrangement occur in these different cases. In some instances a rational explanation of the cause of difference may be given. A familiar example is afforded in the case of the three metamerides, propionic acid, methyl acetate, and formic ether. These bodies are liquids, each of which contains ЄH., they yield vapours of the same density, the latter two even have nearly the same boiling point and specific gravity; but there can be no doubt that they are all differently constituted. Propionic acid is one of the acids of the stearic series. Methyl acetate is prepared by the action of acetic acid upon wood spirit, and acetic acid may be extracted from it by means of an alcoholic solution of caustic potash, whilst wood spirit is liberated; formic ether is the result of an action of formic acid upon ordinary alcohol, and when treated with caustic potash in the same way as the methyl acetate, betrays its origin by yielding formic acid and alcohol. Each of these bodies must therefore be represented as possessing a different molecular constitution, as for example :

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In other cases, such for example as the ordinary sugar of fruits (EH), no reasonable hypothesis of its composition has been offered; yet we know of the existence of several bodies,

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ANALYSIS OF ORGANIC COMPOUNDS.

isomeric with it, such as lactic and acetic acids, for which rational formulæ are in common use.

Even in substances of comparatively simple composition, the difficulty of framing a conclusive theory of their molecular arrangement is very great, as may be seen by inspecting the following table, which represents a few of the views which have been taken respecting the nature of acetic acid :

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H(CH)E0, or HO,(C,H,)C2O, Methyloxalic Acid
H,е(CH), or HO,C,(C,H,O, Methylformic Acid.

. (Empirical Formula.)

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(Liebig.)
(Gerhardt.)

(Kolbe.)
(Wurtz.)

It must however be borne in mind, that conjectures such as these as to the internal arrangement of organic bodies do but represent probabilities. We cannot, indeed, assert positively in any instance what the actual grouping of the component atoms or the relative collocation of the atoms in space really is; but we may infer, with reasonable confidence, that that grouping, be it what it may, is similar in allied or homologous (1058) compounds. The rational formula which embody these views should therefore be regarded chiefly in the light of memoria technica; means by which facts may be riveted upon the memory, and by which analogies that otherwise would escape the notice of the observer may be traced; and, above all, as instruments by which that precision may be given to our ideas which is so essential to the reception and advancement of solid philosophical knowledge.

§ I. ON THE ANALYSIS OF ORGANIC COMPOUNDS.

(1039) A. PROXIMATE ANALYSIS.-In the analysis of organic compounds, two problems are presented to the chemist for solution-the object of the first is to separate the proximate components of the vegetable or animal product from each other; whilst the object of the second is to determine the elementary composition of the proximate principles thus isolated. The separation of wheat flour into starch, sugar, gluten, ligneous fibre, and oily matter, affords an instance of proximate analysis; but the determination of the proportions in which the carbon, hydrogen, and oxygen are united in the component starch, sugar,

PROXIMATE ANALYSIS-DESICCATION-DIGESTION.

or fibre, furnishes an illustration of what is meant by ultimate organic analysis.

The proximate analysis of an organic compound is often a matter of great difficulty. The first process generally consists in the complete desiccation of a given weight of the substance under examination, by exposing it to a temperature of from 212° to 250° in a water oven or box of sheet copper, made double, as shown in fig. 375; a being an aperture for the introduction of oil or of water into the interval between the external and internal plates, the temperature being regulated by a thermometer introduced at b. The loss of weight which the substance under examination experiences during the drying can be accurately ascertained when needed. The dried material is then pulverized, and subjected to the action of several solvents in succession, such as ether, alcohol, and water. A convenient apparatus for the digestion of the substances for analysis in these menstrua, is shown in fig. 376-a is a glass flask containing the liquid to be employed as the solvent : this can be kept in steady ebullition by means of the lamp beneath. B, is a tube of glass or of tin-plate, in the contracted portion of which is a plug of cotton wool, c; in this tube the substance for analysis is to be placed; d, is a short lateral tube to which the tube, e, of glass or of flexible metal is attached. The tube e should be kept warm, by enveloping it in flannel, with a view to prevent the premature con

FIG. 375.

FIG. 376.

A

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[graphic]

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SOLVENTS USED IN PROXIMATE ANALYSIS.

densation of the vapour which rises from the flask.

M,

м, is a tinplate condenser, which can be filled with water; h, is a funnel for conveying cold water to the bottom of this refrigerator, whilst the hot water flows off at the spout above; through the axis of м, passes a tube, g, open at both ends; the lower projecting extremity of this tube is fitted by a cork to the tube B. ƒ is a worm tube, the upper extremity of which passes through the side of the refrigerator, and is adapted by a cork to the tube e, whilst its lower extremity is soldered to the tube g, into which its contents flow after they have been condensed in their passage through the refrigerator. It is obvious that by this arrangement a perpetual distillation of the liquid in the flask A may be readily maintained; the vapour which passes through the tube e becomes condensed in the spiral tube f, and percolates, in the liquid form, through the material contained in B, carrying the soluble matters into the flask A, where they gradually accumulate. If the em

ployment of metal be in any case objectionable, glass vessels may be used, but they are more fragile and more costly than those made of metal.

Ether is particularly valuable as a solvent for fatty substances, and for caoutchouc and camphor; alcohol, for the solution of many crystallizable organic principles, such as the vegetable alkalies; whilst water dissolves sugar, gum, starch, and other highly oxidized bodies which are nearly insoluble in alcohol and in ether. In some cases benzol, in others chloroform, or carbonic disulphide (ES) is a valuable solvent, and may be substituted for ether, which they most resemble in their solvent action. In particular cases dilute acids, and in others dilute alkalies may be employed, but they must be used with caution, since they are liable to act not merely as solvents, but also to produce important chemical changes in the compounds submitted to them. No general rule can be laid down for the extraction of the different proximate principles; each class of substances requiring special modifications, which experience alone can indicate.

In all cases of proximate analysis, the employment of the microscope will afford valuable aid whilst watching the progress of the separation of the various principles, and in enabling the operator to ascertain whether or not the substances which he has isolated are mixed with other bodies which may resemble them in chemical habitudes. When a substance or a deposit assumes the crystalline state, such an examination, by revealing the similarity or difference in form of its component particles, is

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