with a glass plate or with the hand, and the water be then shaken in the jar, the water, owing to its having absorbed the gas, will, when poured out, be found to have acquired the smell and behaviour with litmus and turmeric papers which characterise the gas. This liquid is in fact weak "Liquor Ammoniæ," a solution prepared in large quantities by letting ammonia gas bubble for some time through cold water.

EXP. 25. Pour into a glass jar a little strong hydrochloric acid, close the jar with a glass plate and shake the acid about inside the jar, hydrochloric acid gas will thus be liberated; the liquid may then be allowed to run out by slipping aside the glass plate for a moment. Place this jar in an inverted position over a jar containing ammonia gas, and covered with. a glass plate (fig. 9 a), then withdraw the glass plates, so that the mouths of the jars are in contact (fig. 9b), and the hydrochloric acid and ammonia gases can freely intermingle. Dense white fumes of solid ammonium chloride will immediately be formed:

FIG. 9.

b

NH +HCl=NH C1.

This experiment may also be performed by dipping a glass rod into some strong hydrochloric acid, and holding it in ammonia gas as it issues from the delivery tube of the apparatus, or in a jar previously filled with the gas; the same white

fumes will at once appear.

Tests for Ammonia Gas.-Ammonia gas may be readily recognised by its pungent smell; by turning moistened red litmus paper blue, and moistened turmeric-paper reddishbrown; and also by giving white fumes with a glass rod moistened with strong hydrochloric acid.

Gases are frequently made to pass through certain liquids, in order to free them from impurities before they are col lected; this is termed "washing" a gas. The preparation of

carbon monoxide gas from oxalic acid will serve to show how this process is performed.

V. CARBON MONOXIDE.-This gas is produced when carbon dioxide gas is made to pass over red-hot charcoal :—

CO,+C=2C0.

It is often thus formed in open grates, and is seen burning. at the top with its characteristic blue flame. Carbon monoxide is usually prepared by heating solid oxalic acid with strong sulphuric acid, when a mixture of carbon monoxide and carbon dioxide is given off :

H2C2O4.2H2O+ H2SO4 = CO + CO2 + H2SO4.3H2O.

*

EXP. 26.-Place a little solid oxalic acid in a test-tube; pour upon it strong sulphuric acid sufficient to cover is to a depth of at least half an inch, and heat the mixture. After a short time effervescence will be noticed, owing to gases being evolved; hold in the mouth of the tube a glass rod freshly dipped into lime-water, the drop of lime-water hanging upon its end will become milky, showing that carbon dioxide is one of the gases evolved. Hold a burning taper to the mouth of the test-tube, a blue flame will be seen caused by the carbon monoxide gas burning in the air.

In order to get rid of the carbon dioxide gas which is mixed with the carbon monoxide, the mixed gases are "washed" with solution of caustic soda; this liquid absorbs the carbon dioxide readily, but allows the carbon monoxide to pass on :

CO+CO2+2NaHO = CO + Na2CO3+H2O.

The way in which this washing is effected is explained in Exp. 27.

EXP. 27.-Heat the mixture of oxalic acid and strong

* Strong sulphuric acid is a very corrosive liquid, and great care must be taken not to get any upon the skin or clothes; should any of this acid or of any other acid get upon the skin it must be at once washed off; if it should accidentally be spilt upon the clothes, the part must be rubbed with ammonia solution. If the acid has remained for some time on the clothes it will produce a red stain, which will be removed by ammonia solution unless caused by nitric acid.

sulphuric acid in a flask (A) fitted as shown in fig. 10, and pass the gases either into a wash-bottle (a small Woulffe's bottle, or a broad-necked bottle), fitted as shown in B, and containing caustic soda solution; or through a tube (a) containing fragments of quick-lime, or a U-tube (b) filled with fragments of caustic soda or with fragments of pumice-stone The carbon moistened with strong caustic soda solution. monoxide gas thus more or less perfectly freed from carbon

FIG. 10.

TA

dioxide may be collected over water, and will be found to give either no milkiness with lime-water, or a slight milkiness if the stream of gas has been so rapid that the caustic soda has not been able to absorb the carbon dioxide completely. By heating a formate with strong sulphuric acid carbon monoxide alone is given off, and is thus readily obtained free from carbon dioxide.

Carbon monoxide resembles hydrogen in being inflammable and in extinguishing a burning taper; it also explodes when mixed with oxygen or air, hence before collecting a cylinder of the gas for experiment, ascertain that the gas coming off from the apparatus is free from air by collecting a small testtube full and proving that it burns quietly.

EXP. 28.-Push a burning taper up into a cylinder filled with carbon monoxide, the gas will burn with a blue flame at the mouth of the jar but the taper will be extinguished. As soon as the gas has ceased to burn inside the cylinder, pour in a little lime-water and shake it about; the liquid becomes milky, showing that by the combustion of carbon

monoxide in the oxygen of the air carbon dioxide gas is produced :

CO+O=CO,.

Tests for Carbon Monoxide.-Carbon monoxide is recognised by burning with a pale blue flame in the air, producing carbon dioxide, which renders lime-water milky.

V. a. Chlorine Gas may be made in the apparatus employed for the preparation of carbon monoxide the washing-bottle may either be dispensed with or may be used containing a little water.

:

EXP. 29.-Place some manganic oxide, powdered or better in small lumps, into the flask A (fig. 10, p. 22), pour upon it some strong hydrochloric acid mixed with about one-third its measure of water, and heat gently in a draught-cupboard or out of doors in the open air. A greenishyellow gas is evolved, which may be collected by displacement like carbon dioxide, since it is much heavier than air :

MnO2+4HC1=Cl2+ MnCl2 + 2H2O.

The gas has a very destructive action on the lungs, and must on no account be inhaled; it is usually recognised by its yellowish-green colour, its peculiar smell, and by its property of bleaching moist vegetable colours. This last property is shown by placing in a jar of the gas a piece of moistened litmus-paper or fabric dyed with madder ("Turkey red"): the colours of both will be destroyed. A burning wax-taper plunged into a jar containing chlorine continues to burn with a very smoky flame oil of turpentine, introduced into the gas by moistening a strip of filter-paper with the warm liquid, catches fire of itself and gives rise to dense smoke. A piece of Dutch-foil or copperleaf also burns when dropped into chlorine.

Tests for Chlorine.-Chlorine gas is recognised by its yellow colour, its smell, and its power of bleaching moistened litmus paper.

V. b. Hydrogen Chloride, or Hydrochloric Acid Gas.

EXP. 30.-Place in the cleansed flask A (fig. 10), which was used for the preparation of carbon monoxide, some lumps of sodium chloride (common salt), obtained by breaking up a mass of the melted powder, or by breaking a piece of rock-salt; pour upon it strong sulphuric acid and heat gently. Hydrochloric acid gas is evolved :—

NaCl + H2SO4 = HCl +NaHSO1,

and being heavier than air, may be collected by displacement in the same way as carbon dioxide. The gas fumes strongly in moist air, turns moist blue litmus-paper red, dissolves easily in water (Exp. 24) giving an "acid" liquid (hydrochloric acid), which, like the gas, turns blue litmus red.

Tests for Hydrochloric Acid.—This gas is known by fuming in the air, turning moist blue litmus red, giving white fumes with ammonia gas, and yielding when dissolved in water a milky liquid on addition of silver nitrate solution which does not become clear on adding nitric acid.

VI. DISTILLATION.-This process is employed to separate liquids which boil at a comparatively low temperature, either from solids, or from other liquids which are not converted into vapour at all or only at much higher temperatures. It consists in boiling the liquid and cooling (" condensing ") the vapour, which is thus given off, again into a liquid (the distillate"), the non-volatile solid or liquid substances present being thus left behind in the vessel in which the liquid is boiled. As examples of this process, the purification of common spring-water from the solid substances dissolved in it, and the preparation of nitric acid, may be performed.

66

A. Distillation of Water.-EXP. 31.-Pour into a clean retort a (fig. 11) some tap-water through a funnel placed in the mouth, or in the tubulure (i.e., the opening for the stopper or cork) if, as is better, a tubulated retort is employed. Support the retort, whose bulb has been about halffilled with water, in a retort-stand or upon a tripod, with its neck sloping downwards and dipping into a small clean flask or bottle b, which is partly immersed in cold water contained in an evaporating basin. Cover the bulb of the flask with a broad strip of filter-paper whose ends dip into the water contained in the dish; or instead of cooling the flask cool the neck of the retort, by wrapping round the lower part of it a piece of filter paper c, and round this a piece of wet string or tow d; then arrange a funnel e with its mouth partly stopped so as to drop cold water on the upper part of the filter paper; this water will be drained off by the string d. On carefully boiling the water in the retort steam passes into the flask and is there condensed to "distilled water." The first few drops should be thrown away, as they are apt to be impure from rinsing the retort neck and flask. Whilst this process of distillation is go