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some sodium chloride dissolved in water so that when removed a drop of the solution remains in the loop; then hold the loop at the tip of the inner blowpipe flame, the outer flame will be coloured bright yellow. Repeat the experiment, placing the moistened loop in the upper and outer part of the Bunsen flame, the same yellow tint will be obtained. This yellow colour is given only by sodium and its compounds.

Remove all sodium chloride from the platinum-wire loop by heating it strongly in the tip of the blowpipe flame until it ceases to impart any colour to the flame. Then dip the wire into some potassium chloride solution: this will give to the flame a pale violet colour, which is characteristic of potassium and its compounds.

As will be seen subsequently, the examination of flame colorations, aided by the use of coloured glasses and of a hollow glass prism filled with indigo solution, is of great use in ordinary analysis.

EXP. 49 a.-Look at the yellow flame coloration produced by sodium chloride through a piece of cobalt-glass or through the indigo-prism, no yellow colour will be visible. Now dip the wire-loop into a mixture of a few drops of sodium chloride and potassium chloride solutions, when held in the flame only the sodium coloration is seen, but through the blue glass or prism the potassium is seen to produce a red flame coloration. By means of the blue glass or prism then potassium is discoverable in the presence of sodium, by which its presence is otherwise masked.

By far the most perfect method of examining flame colorations, more especially for the rarer elements or for mere traces of the more common ones, is by the use of the spectroscope; every advanced student should make himself familiar with the use of this instrument, at least in its simplest and smallest form. A paper on the use of the spectroscope for qualitative analysis will be found in the appendix; this is well worthy of the attention of senior students.

It is well to keep two platinum wires, one to be used for borax beads and the other for flame colorations, since a loop which has been used for borax colours the flame yellow.

33. Ignition on Charcoal in the Blowpipe Flame.-Common wood charcoal forms a very useful support for substances which are to be heated in the blowpipe flame. It cannot itself be melted, it conducts heat very badly, and possesses other properties which render it peculiarly adapted for ignitions. It is, if necessary, sawn into small pieces of the required shape and size with a fine-toothed saw.

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Ignition on charcoal serves chiefly to detect the presence of metals; the substance for this purpose is heated either alone or mixed with certain other substances (usually sodium carbonate and potassium cyanide) in the inner blowpipe flame; globules or scales of the metal are thus frequently produced, also in many cases the oxide of the metal forms a characteristic coating or "incrustation" on the charcoal an examination of the colour and properties of the metal or incrustation or of both, will frequently lead to the detection, or confirm the presence, of some metal. Charcoal is apt to leave a white ash when heated alone in the blowpipe flame; this must not be mistaken for an incrustation. A powder frequently requires to be moistened with water before being heated by the blowpipe flame, in order to prevent the flame from blowing it off the charcoal.

EXP. 50.-Choose a piece of wood-charcoal free from large cracks, scoop out a small hollow near one end with a knife,

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some time bright globules of lead will be seen in the cavity, and on removing the charcoal from the flame the surface round the cavity will be found to be coloured with a yellow

"incrustation."

If one of the little globules of metal be then taken off the charcoal with the point of a penknife, and be struck smartly with a pestle on the bottom of a mortar turned upside down, it will flatten out into a cake, showing that the metal is "malleable" not "brittle." The metal lead is the only metal which gives a yellow incrustation and malleable globules; this test therefore serves to detect its presence.

33 a. The melted mass left on the charcoal after fusing a metallic salt with sodium carbonate is best examined for Fig. 32.

metallic powder or scales by detaching it from the charcoal with the point of a penknife, then crushing it in an agate mortar or in a watch-glass with a little water; on stirring and quickly decanting the water down a wet glass rod or pestle held against the edge of the vessel (fig. 32), the metallic powder, if present, will be left; by repeating this process of "levigation," the metal is obtained free from soluble substances and charcoal, it may then be further examined by a lens or magnet. A mixture of copper sulphate with sodium carbonate, when heated on charcoal and "levigated" as directed above, will furnish little red scales of copper: a mixture of ferrous-sulphate and sodium carbonate will yield a grey powder consisting of metallic iron, which, when touched with the pole of a magnet, will become attached to it.

34. Use of Test-papers.-Certain vegetable colours undergo remarkable changes when acted upon by chemical substances. Some of these are used for analytical purposes; the following experiment will explain how they are made use of.

EXP. 51.-Boil some pieces of a common red pickling cabbage-leaf in water; a blue liquid will be produced. Pour some of this liquid into a test-tube and add several drops of hydrochloric acid, the colour will change to bright red; to another portion of the blue liquid add several drops of ammonia, the colour changes to green; to a third portion of the blue liquid add a little ammonium chloride solution, the colour remains unchanged.

Mix in a test-tube a little ammonia solution with three times as much distilled water, and add this weak ammonia gradually to the liquid reddened by several drops of hydrochloric acid; by careful addition the blue colour may be exactly restored, the acid previously added being "neutralised" by the ammonia.

EXP. 52. Pour some blue litmus solution (made by boiling solid litmus in water and filtering) into three test-tubes; to the first add hydrochloric acid drop by drop until it turns distinctly red, preserve this liquid; to the second add ammonia solution, the liquid remains blue; to the third add ammonium chloride solution, the colour will not be affected.

Divide the reddened liquid into three parts; to one part add ammonia solution, the original blue colour is restored; to another part add hydrochloric acid, the red colour is unchanged; to the other part add ammonium chloride, the red colour remains unaltered.

These experiments may also be made with litmus testpapers, which are prepared by soaking paper in the blue and red liquids respectively and allowing it to dry.

EXP. 53. Repeat the preceding experiment, using the litmus papers instead of the solution, and moistening them with the liquid by dipping a glass rod into it and then touching the paper with the moistened end.

It will be seen then that the three substances employed differ in their action on litmus test-papers.

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Each of these substances is a sample of a whole class, the members of which resemble it in their action on litmus.

The class of acid substances, including sulphuric acid, nitric acid, &c., and many metallic salts, resemble hydrochloric acid in reddening blue litmus and not affecting red.

The class of alkaline substances, including potassium hydrate, sodium hydrate, &c., and many salts of the alkalis, resemble ammonia in blueing red litmus, and not affecting blue.

The class of neutral substances, including potassium chloride, sodium chloride, &c., resemble ammonium chloride in not affecting either blue or red litmus.

Hence in analysis we use red and blue litmus-papers to test whether a liquid is "acid," "alkaline," or "neutral.”

The red cabbage liquid, as will be seen, is better suited for a test-paper than litmus, since one paper shows both alkalis and acids; but the colour is very apt to spoil with keeping.

Sometimes a liquid in which a coloured precipitate is suspended has to be tested by litmus-paper, the precipitate then hides the colour of the paper when a drop of the liquid is placed upon it. In such a case moisten the paper with a drop of liquid, and wash the liquid off after a short time with a stream of distilled water from the wash-bottle; the colour of the paper is then distinctly seen.

35. An acid substance by being mixed with a proper proportion of an alkaline substance becomes "neutral;" similarly an alkaline substance is rendered "neutral" by mixture with a proper quantity of an acid. Thus by mixing hydrochloric acid and ammonia in the proper proportions, the neutral substance ammonium chloride is formed; the acid is then said to be "neutralised" by the alkali, or the alkali by the acid.

It is often required to "neutralise" an acid with an alkali or vice versâ; the following experiment will show how this process of neutralising an acid is performed.

EXP. 54. Pour a little hydrochloric acid into a porcelain dish, add to it ammonia several drops at a time keeping the liquid constantly stirred with a glass rod, and occasionally touching a slip of blue litmus-paper with the end of the rod

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