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Analysis of Subdivision IV.

The acid-radicals of more common occurrence only being included, the salt may be a NITRATE, PHOSPHATE, ARSENITE, or ARSENIATE.

Evidence of the presence of the first of these acid-radicals may be obtained by adding concentrated sulphuric acid to the solid, or its strong solution, and heating.

Pungent orange brown vapours indicate a nitrate. Proof of the presence of arsenic will have been obtained in the examination for the basic radical, which in analysis always precedes the search for the acid-radical. If the salt, when heated with carbonate of sodium and charcoal in a bulb tube (see p. 210), yields

a metallic mirror, an arsenite or arseniate is indicated.

Further Analysis. Acidify a portion of the solution of the salt with a few drops of dilute sulphuric acid, and pass hydrosulphuric acid gas.

A yellow precipitate of If no precipitate, or only a white one of

As, S, or As, S. sulphur (due to nitric acid), warm the solution, would indicate the to expel every trace of hydrosulphuric acid; presence of

| add excess of acetate of potassium, and a drop As, O, or AsOy or two of perchloride of iron. To distinguish between

these, the silver test must be resorted to, being A white pre- ! If no precipitate, a fresh

applied to a perfectly I cipitate of portion of the original soluneutralized part of the

Fe, PO, tion of the salt should be original solution. would indicate the mixed with concentrated Ag,As, O, is yellow; I presence of sulphuric acid (the testAgzAso, is brick-red.

PO, tube being cooled at the

time of mixture by immersion in water), a crystal of ferrous sulphate then added, and the whole allowed to rest: the formation of a reddish brown halo around the crystal, consisting of 4Fe,SÓ, N, O,, would indicate the presence of

NO.

385

Part II.—THE METHOD OF ANALYSIS.

CHAPTER I.

INTRODUCTION.

TERRESTRIAL matter, although consisting primarily of a definite number of elements, is found, as the student will now have learnt, in an almost infinite variety of combinations. These combinations or salts are formed, it will be remembered, by the union of two classes of bodies, which may be either simple or compound, and are distinguished as basic radicals, and acidradicals. To test these combinations, that is, to identify their basic or acid constituent, is easily accomplished by the employment of any appropriate reaction which may elicit some characteristic feature of colour, odour, insolubility, &c.; but to analyse such a combination, that is, to separate its basic and acid-radical either in an isolated condition or in a new form, is a more difficult task. And this difficulty is, of course, enhanced when the process of separation has to be performed upon a complicated mixture of salts.

Although complex bodies of mineral origin, almost without exception, as well as a vast majority of those derived from vegetables and animals, are reasonably believed to be definite saline combinations of basic and acid-radicals, still several substances which occur abundantly in organic structures, such as albumen, fibrine, starch, and gelatine, are composed of molecules so complex as to baffle all attempts made to catch a glimpse only of their constitution. These bodies, therefore, are not subjects for chemical analysis; at least, they cannot be resolved, like the saline combinations of compound radicals, into their proximate, but only into their several elementary constituents.

Since, therefore, the processes of qualitative analysis to which the present work is confined have for their object the separation, recombination, and recognition of certain forms of elementary or of definite compound matter, i.e. the separate identification, in the bodies termed salts, of their acid- and basic radicals, it is obvious that all indefinite compounds, to which ultimate analysis only is applicable, are excluded from consideration. All salts commonly met with (using the term salt in its widest sense, as including bases, oxides, acids, &c.) may, however, be considered as fit subjects for qualitative chemical analysis.

It will be at once seen that the certainty of analysis depends much upon the simplicity or complexity of the bodies with which it has to deal. Thus, chemical analysis is most likely to be successful with bodies of mineral origin ; for in them, generally speaking, the elements exist in the simplest forms of combination. These constituent elements of mineral substances are, moreover, possessed of most powerful chemical affinities, and, above all, are usually neither themselves gaseous at ordinary temperatures, nor do they yield gaseous products. On the other hand, the bodies derived from the animal and vegetable kingdoms are compounds in which, for the most part, the four elements, carbon, hydrogen, nitrogen, and oxygen, exist in large proportion. The ready decomposability of these organic bodies depends in great measure upon the gaseous character, i.e. the volatility of their constituent elements, and upon the remarkable tendency which carbon and hydrogen, or carbon, hydrogen, and oxygen present, to unite in endless and ever-varying proportions. Other causes combine to render these substances unstable, such as the great affinity of carbon and of hydrogen for oxygen (CO, and H, O being thus formed), and of nitrogen for hydrogen (NH, being the product): the gaseous character or ready volatility of the simpler compounds thus produced is also to be taken into account. But the causes of the instability of these organic substances is not now the object of discussion ; it is merely adduced to show how small a hold upon them chemical analysis possesses, when we consider that its object is to separate a particular com

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pound, which in these substances may slip, by a Protean metamorphosis, through the fingers of the experimenter in the very process of detection. The action of reagents, themselves the means of analysis, is sufficient to decompose bodies such as these.

But it is far otherwise with the elementary or simpler forms of matter which constitute mineral bodies. These are not so affected by reagents; and if isolated, there is in most cases no danger of their volatilization at ordinary temperatures, and none of their decomposition : neither is their presence so concealed as to be undiscoverable ; for no known means, however energetic, can destroy their identity. It matters not whether they exist dissolved in mineral waters, crystallized as distinct substances in masses of mineral, or diffused in minute quantity through various soils; their recognition and separation is a matter of almost equal certainty. It is thus that mineral poisons are so much more easy of detection than those of vegetable or animal origin. The latter poisons are, in the majority of cases, easily destroyed, and often cannot be satisfactorily pronounced to be the poison supposed without a quantitative determination of the proportions in which the carbon, hydrogen, nitrogen, and oxygen are present. Arsenic, lead, and copper, on the other hand, are subject to no such accidents. Whatever stage of decomposition the body poisoned by these substances may have reached, these minerals can still be recognized and separated from the mouldering remains.

So far for the limits and the scope of qualitative analysis ; we will now speak of its method.

The student will now have become acquainted with a certain number of tests, the application of which, either singly or combined, cannot fail to assure him of the presence or absence of every basic or acid-radical. To analyse successfully, there is, however, one point to which he must invariably attend; and that is, to preserve a well-ordered sequence in the application of the tests at his disposal. By the tables which have been appended to each subdivision of the basic and acid-radicals (in Chapters

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