obtained “anhydrous" by digestion with, and distillation from, caustic lime (CaO). For most purposes this is, however, unnecessary. The specific gravity of anhydrous ether is 0.73.

Hydrate of ethyle or alcohol ([C, H2]HO).

This is to be obtained of great purity as "rectified spirit," having the specific gravity of 0.83; this, like commercial ether, contains water, from which it, too, may be freed by distillation with caustic lime. By other subsequent distillations with ignited carbonate of potassium (K, CO,), or anhydrous sulphate of copper (Cu2 SO), it may be rendered quite free from water. It is then termed "absolute," and has a specific gravity of 0.795. For all ordinary purposes the alcohol recently introduced by the Excise, and known as "methylated spirit," may be employed. This spirit is simply alcohol intentionally and avowedly mixed with methyle alcohol to the extent of 10 per cent. This is done in order to preclude its use as a beverage, without interfering with its applicability to most manufacturing and chemical purposes.

SALTS OF HYDROGEN.

Chloride of hydrogen or hydrochloric acid (HCl).

Nitric and sulphuric acids and salts of iron are common impurities of commercial hydrochloric acid: it may, however, always be obtained of great purity in commerce. Its specific gravity should be 1.2. 1 part, by measure, of acid diluted with 2 parts of water is often used in analysis as dilute hydrochloric acid.

Nitrate of hydrogen or nitric acid (HNO,).

The commercial acid generally contains a little hydrochloric and sulphuric acid (H2SO4); it may, however, be obtained pure. The diluted acid should have the specific gravity of 1·12, and is made by mixing 1 volume of strong acid with 2 of water.

Acetate of hydrogen or acetic acid (HC2H ̧O2 or HÃ).

2

This acid as obtained in commerce is of sufficient purity for most analytical operations. It should have a specific gravity of 1048. It occasionally contains sulphuric acid.

Oxide of hydrogen or water (H2O).

This substance, as it commonly occurs, holds various salts in solution, of which the principal are the chlorides, sulphates, and carbonates of potassium, sodium, calcium, and magnesium: from these impurities it may be separated by distillation.

Sulphide of hydrogen or hydrosulphuric acid or

sulphuretted hydrogen (H, S).

This substance may be easily prepared by acting upon almost any metallic sulphide with hydrochloric acid or with sulphuric acid; in practice, however, the protosulphide of iron (Fe, S) is almost invariably employed: the evolved gas is washed by passing it through a wash-bottle containing a

small quantity of water, and it may be afterwards dried by passing it through a tube containing chloride of calcium in small fragments. The solution of this gas in water may be prepared by passing the gas into distilled water as

Fig. 6.

long as it is dissolved, which may be ascertained by observing whether the bubbles as they pass through the liquid are dissolved, or whether they escape without diminution: a saturated solution of this gas is much employed in analysis.

Sulphurous acid (H2 SO,).

This acid is not known to us in the separate state, but only as a solution of the compound called sulphurous acid gas (SO2) in water (H2O). Sulphurous acid gas is evolved when charcoal (C), copper (Cu), or mercury (Hg) are boiled with concentrated sulphuric acid. The following changes

occur:-

2(H,SO,)+C=2(H,SO,)+CO,

2(H2SO1)+2Cu=H2 SO2+Cu2 SO4+H2 (

but the sulphurous acid splits thus--

3

H2SO, H2O+SO2.

3

0:

Sulphurous acid gas may be washed, and water may be saturated with it, just as in the case of ammonia gas.

Sulphuric acid (H2 SO1).

The common impurities of the commercial acid are lead (Pb) and arsenic (As); both are separated by passing a stream of sulphuretted hydrogen through the diluted acid. For ordinary purposes a good specimen of commercial acid suffices, but for special cases, such as the detection of poisons in medico-legal investigations, it is necessary to employ an acid which is absolutely pure. The dilute acid employed in analysis is prepared

by mixing 1 part of the concentrated acid (oil of vitriol) with 5 parts of water; the greater part of the lead is separated by this dilution.

Carbonic acid (H2 CO2).

2

This acid is not known to us in the separate state, but only as a solution of carbonic acid gas (CO2) in water (H2O), for H, CO,=CO2+H2O. The gas (CO2) can be easily prepared by decomposing any carbonate by an acid in an apparatus similar to that figured on page 64. Carbonate of calcium (marble) and hydrochloric acid (HCl) are usually employed:

Ca2 CO2+2HCl=H, CO2+2CaCl;

but the carbonic acid is immediately decomposed into water and carbonic acid gas, which may be passed into water, in which a portion will dissolve, or into the solution to be submitted to its action.

The commercial substance is of sufficient purity for ordinary operations. It may be purified by sublimation. 1 part of crystals should be dissolved in 20 of water.

Sulphindigotic acid.

The solution of indigo in oil of vitriol, or strong sulphuric acid, is called by this name. It is used in a very dilute state as a chemical test.

This acid is met with of sufficient purity for almost all analytical purposes. It may be purified by recrystallization.

Hydrofluosilicic acid (H, Si, F,).

Fig. 7.

This acid is prepared as follows:-A mixture of 1 part of sand (Si2 O3) and I part of fluoride of calcium (CaF) is introduced into the flask A; 6 parts of concentrated sulphuric acid are then added, and heat applied by means of a sand-bath. A glass delivery-tube passes through 4 parts of water placed in the jar B, and dips beneath the surface of the mercury, C, at the bottom of the vessel. The following actions take place:-Hydrofluoric acid (HF) is generated by the action of the sulphuric acid on the fluoride of calcium (CaF), and in its turn this hydrofluoric acid acts upon the sand present, producing gaseous fluoride of silicon; thus

2(CaF)+H2SO1=2(HF) +Ca2 SO1; and 6(HF) +Si ̧ O ̧ =3(H2O)+2(SiF,)

Fluoride of silicon.

B

The fluoride of silicon escapes through the mercury, C, into the superincumbent layer of water, and by the latter it is instantly decomposed, silicic acid

being again formed and precipitated, while the hydrofluosilicic acid remains dissolved in the water; thus

6(SiF,)+4H, 0=2(HSiO2)+2(H,Si, F,).

The use of the mercury in preventing the access of water to the mouth of the delivery-tube is obvious: the liquid should be filtered through a linen cloth to separate the gelatinous silicic acid, and the filtrate preserved in a bottle of German glass.

ACID ELEMENTS.

Carbon (C).

The charcoal selected for blowpipe examinations should be made of sound beechwood, and should be free from bark or knots. If the pieces are about 1 inch in diameter, they should be sawn into pieces of about 4 inches in length, and each piece should be divided longitudinally, the flat surfaces of the section being well adapted for blowpipe experiments.

Bromine (Br).

This salt-radical may be obtained in commerce of sufficient purity.

Chlorine (Cl).

Chlorine may be easily prepared by one of the methods given on p. 19; if required dry it may be passed over fragments of fused chloride of calcium (CaCl) in a long tube, or over pieces of pumice-stone soaked in oil of vitriol.

TEST PAPERS.

Vegetable blues, or at least most of them, possess the peculiar property of becoming red when moistened with an acid, i. e. the hydrogen salt of a simple or compound acid-radical, while their original colour is restored by an alkaline solution, that is, by the solution of a substance whose basic properties are definite. Some of the most delicate vegetable blues even assume a new colour when submitted to an alkaline liquid, becoming a brilliant green, whilst vegetable yellows, when dipped into alkaline solutions, become redbrown, but are not influenced by acids beyond the restoration of their original colour (boracic acid being an exception, for it behaves like an alkali). These indications, although very valuable, must not be too implicitly relied on, since certain salts which are theoretically neutral produce changes of colour.

Blue litmus paper.

The litmus of commerce should be dissolved in water, and very dilute sulphuric acid added to the clear blue solution until the colour has been changed to a reddish violet; the blue colour is then restored by the addition of a small quantity of the original solution: white writing-paper, not highly glazed, is then to be painted with the blue liquid, on one side only, and the coloured pieces, when dry, are to be cut into narrow strips for use, and preserved in a well-stoppered bottle.

Red litmus paper.

This may be prepared by using the blue liquid just mentioned, after having slightly reddened it with a drop of very dilute sulphuric acid.

Dahlia paper.

If the richly-coloured petals of the purple dahlia, or those of the heartsease, are boiled with alcohol, a red solution is obtained, which is of far greater delicacy than that produced by common litmus; it will become green also by the action of alkalies. Paper may be coloured with it as with the litmus solutions.

Turmeric paper.

An alcoholic extract of turmeric-root is of an orange-yellow colour, and becomes reddish brown when submitted to the action of alkaline solutions. Manganese paper.

Strips of paper dipped in a moderately strong solution of manganous sulphate (Mn, SO) are occasionally employed for the detection of ozone.

Acetate of lead paper.

Strips of paper steeped in a solution of acetate of lead (PbC, H, O2) are very useful in the detection of sulphuretted hydrogen (H, S).

Starch paper.

Strips of paper dipped in solution of starch made by boiling starch in water, and kept somewhat moist, are very useful in the detection of bromine and iodine.

ORGANIC BODIES.

Those forms of these bodies found in commerce may be safely employed, selecting, of course, the uncoloured variety of Starch (C12 H20 O10); the purest white Sugar (C12H22O11), and that kind of Gelatine known as isinglass.

CHAPTER VI.

DETECTION OF THE BASIC RADICALS IN THEIR
COMPOUNDS.

We have now introduced to the notice of the student the characteristic features of the basic and acid elements, existing as elements in the uncombined state, and have remarked upon the peculiar properties which they manifest, and by means of which their identity can always be safely established. The consideration, however, of the laws of chemical combination will have