with a slight report. The gas has ascended from the bottle into the tumbler, and is consequently lighter than common air. In this experiment the lower vessel must not be immediately exposed to the lighted taper, because, if all the hydrogen is not displaced, an explosion might ensue that might break the bottle; but if the taper be applied after ten minutes have elapsed, the bottle will be found no longer to contain any combustible gas, the gas having entirely escaped. Fig. 40. Hydrogen is the lightest of all gases. Its specific gravity is 1, and 14 measures of it weigh only as much as one measure of atmospheric air. On account of this lightness, it may be used for filling balloons. Experiment 8.-If, instead of the glass tube, a piece of tobacco-pipe be adapted to the cork of the flask from which hydrogen was evolved, and the gas then lighted, it will burn like a taper. To kindle the gas, instead of a match or a taper, very finely divided platinum may be employed. This can be prepared in a few minutes by dropping a solution of platinum chloride on blotting-paper, attaching it to a wire, and igniting it over a spirit-lamp, till nothing but a grey coherent ash remains. The platinum is thus reduced to an extremely minute state of subdivision, and in this state it exhibits the remarkable property of igniting in hydrogen and inflaming it. It is called spongy platinum, and is employed as tinder in the well-known Döhbereiner's lamp. The apparatus here represented consists of a flask, having the bottom broken off, and to the neck of which the cover of the glass vessel, c, with the cock, e, is fastened airtight. A piece of zinc is suspended in the flask by means of a wire. If diluted sulphuric acid is now poured into the vessel, upon which the cover with the flask attached is placed, then, the cock being opened, that the air contained in the flask may be displaced by the acid from beneath, hydrogen is immediately evolved by the contact of the zinc with the acid, which hydrogen must be collected in the flask by closing the cock, e, the acid being thereby forced into the exterior vessel, until it no longer touches the zinc. Upon opening the stop-cock, e, the gas issues from the fine jet, and is directed against the spongy platinum, f. As the gas escapes, the sulphuric acid passes again into the interior vessel, and gene Fig. 41. rates fresh hydrogen upon reaching the zinc. Spongy platinum possesses, in a high degree, the power of absorbing oxygen and condensing it within its pores; if hydrogen be then presented to it, these two gases will be brought into such intimate contact, by the powerful force of attraction, that they will chemically combine to form water, and the heat thus liberated is sufficient to ignite the platinum tinder, and to inflame the gas, which subsequently issues from the jet. Many aeriform bodies, which do not freely unite with each other, can be forced to combine by means of spongy platinum. Experiment 9.-To observe the remarkable lightness of hydrogen, a small balloon of gold-beater's skin may be filled with hydrogen by means of the apparatus used in Experiment 5. The balloon is squeezed flat to expel the air, and the tube delivering the gas is passed a short way into its orifice, and secured there with a piece of thread. When the inflation is complete, the tube is withdrawn, and the twine tightened simultaneously, thus preventing any escape of gas. The balloon, if unimpeded, will then ascend to a great height. It may be made captive with a piece of thread. Experiment 10.-Pour the contents of the flask in which the hydrogen was generated (Experiment 5) into a porcelain dish, boil until they are reduced in bulk to one-half or thereabouts, and filter them (page 49). A black residue will remain on the filter, which consists of the impurities contained in the zinc; the zinc itself has been dissolved, and has been converted into a salt (page 97), called zinc sulphate, which, on the cooling of the solution, is deposited in colourless crystals. The reaction which takes place between zinc and sulphuric acid is represented by the following equation: Sulphuric Acid. Zinc. H2(SO) + Zn = Zinc Sulphate. Hydrogen. Chlorine is one of a group, the members of which are characterized by a remarkable similarity of chemical properties. It consists of the elements chlorine, bromine, iodine, and fluorine, the three first of which are often termed halogens (from aλs, sea-salt), in allusion to their marine origin. Chlorine is a greenish-yellow gas, 2 times heavier than air, suffocating and irrespirable unless very much diluted with air. Its odour, when very dilute, is somewhat like that of sea-weed, a peculiarity which it shares with the other halogens. Chlorine occurs only in combination; chiefly with sodium as common salt (sodium chloride, NaCl), which forms immense deposits in England and elsewhere, and is the chief ingredient of sea-water. PREPARATION. Experiment 1.-Pour one ounce and a half of hydrochloric acid upon a quarter of an ounce of finely-powdered black oxide of manganese, and heat it gradually in a flask, to which Fig. 42. is adapted a bent glass tube; a yellowish-green gas is disengaged, which is collected by the process already described. The pneumatic trough is, however, filled with warm water instead of cold. This gas is chlorine (from xλwpós, green). Fill with it several six-ounce bottles of white glass, and cork them up. Fill, likewise, a bottle with two-thirds of chlorine and one-third of water, and shake it up; suction is exerted upon a finger which closes the mouth of it,-a proof that a vacuum has been produced. If the finger be removed, the air immediately rushes in. This vacuum was caused by the chlorine having dissolved in the water, which might be inferred also from the disappearance of the yellow colour from the upper part of the bottle. One measure of cold water dissolves two measures of chlorine. This solution is called chlorine water. The mode in which chlorine is formed in this experiment is shown in the following formula : MnO2 + 4 HCl = Mn Cl2+ 2H2O + Cl2. When the evolution of gas has quite ceased the liquid in the flask may be filtered and evaporated, when it will yield on cooling pink crystals of MnCl. Experiment 2.-Chlorine may also be prepared from common salt by mixing three quarters of an ounce of it with half an ounce of black oxide of manganese, two ounces of sulphuric acid, and one ounce of water, and heating the mixture: Sodium Manganese Sulphuric Sodium Manganese 2 NaCl+MnO,+2 H2SO, Na2SO,+MnSO,+2H,O+Cl2 = Chlorine acts as a poison on being inhaled; hence, care - must be taken not to inhale it while preparing it. For greater security, pour some drops of alcohol and ammonia upon a cloth and wave it frequently in the air; the chlorine contained in the air will then be so altered that it will lose its injurious properties. PROPERTIES. Experiment 3.-In order to recognise the odour of chlorine, smell chlorine water (but not the gas) cautiously; the chlorine water may be tasted also without danger. Experiment 4.-If a flask containing chlorine gas be exposed to the air for a short time, no diminution of the chlorine will be perceptible; but if the flask be inverted it will soon contain only atmospheric air. Chlorine is two and a half times heavier than common air, and may be easily poured from one vessel to another like water without material waste; its specific gravity is 35.5 (H=1). Experiment 5.-Introduce a piece of litmus-paper into chlorine gas, and it becomes white; pour chlorine water upon red wine, or ink, and both the liquids will lose their colour. Chlorine bleaches and destroys most colours derived from the animal or vegetable kingdom. In consequence of this property, chlorine has become a most important agent in bleaching; and linen, cotton, paper, and other materials, may be rendered perfectly white by it in a few hours; while, by the old method of laying them on the grass in the sun, weeks, and even months, were required for effecting it. Substances called antichlors are sometimes used to remove the last traces of chlorine. Sodium hyposulphite is the most powerful. The modern method of bleaching is very excellent, and does not in the least injure the strength of the fabric, provided all the chlorine be completely removed after the bleaching is finished, which is not so easily done as might be supposed. If this precaution is not observed, or if the chlorine water is too strong or in excess, then, indeed, after the colour is destroyed, the fibres of the yarn or fabric itself will be attacked. The substance commonly called chloride of lime is now used instead of chlorine. It is a salt from which chlorine is easily discngaged, even by mere exposure to the air. Experiment 6.-Apply chlorine water to decaying and nauseous substances (water in which flowers have been kept, manure, rotten eggs, &c.); the bad odour will at once entirely vanish. Thus it not only decomposes colours, but also the volatile combinations formed during putrefaction, and which occasion disagreeable odours. It acts in a similar manner also upon morbific matter (matters of contagion, miasmata), which, being diffused in the air or attached to clothes and beds, may communicate disease. Chlorine is, therefore, a powerful disinfecting agent, and is used for purifying all putrefying matter and infected atmospheres, and for arresting the decay of organic substances. Musty casks may also be purified by washing them first with chlorine water, and then with some milk of lime. Mouldy cellars, in which milk or beer cannot be kept without turning sour, are again rendered serviceable for a long time by fumigating them with chlorine gas, or by washing them with chlorine water, or a solution of chloride of lime. Experiment 7.-Fill a small bottle with chlorine water, and invert it in a vessel filled with water; if this is put away in a dark place, it remains unchanged; but if it is exposed to the sun, a colourless gas will collect in the upper part of the |