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Hydrogen gas is the lightest substance known, being fifteen times lighter than atmospheric air. It is colourless and transparent, incapable of supporting combustion or respiration, but is itself combustible. Hydrogen, as its name implies (being derived from two Greek words, signifying the generator of water), is a const.i.tuent of water in the proportion of one-ninth by weight, and is always obtained by decomposing that fluid, by presenting to it some body to take up its other ingredient, oxygen, and so set the hydrogen at liberty. If the steam of water be pa.s.sed through a red-hot gun barrel, containing iron filings, the water is decomposed, the iron taking the oxygen, and the hydrogen comes over in torrents; but as every one has not a gun barrel and furnace to heat it, the usual mode is to employ dilute sulphuric acid, and iron filings, or zinc, in small pieces, and it may be collected over water by means of a bent tube issuing from the bottle in which it is formed. It is so light that it was used to fill balloons before coal gas was to be had, and if you procure a light air-tight bag of silk, or thin membrane such as a turkey's crop, and fill it with the gas, it will ascend rapidly, and dance about the ceiling of a room.

EXPERIMENTS.

1. Attach a tobacco-pipe to a bladder filled with this gas, and blow some soap-bubbles with it; they will rise very rapidly, and if a lighted taper be applied to them they burn.

If you mix in a soda water bottle one-third of oxygen with two-thirds of hydrogen, and apply flame, the mixture will explode with a sharp report.

Great care must be taken in all experiments with the mixed gases. To avoid danger the gases are placed in separate india-rubber bags, and are only brought together at the jet. This is an expensive apparatus, and should only be used by experienced persons.

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2. If a jar of this gas be held with its mouth _downwards_, and a lighted taper pa.s.sed up well into the jar, the taper will be extinguished, and the gas take fire, and burn quietly at the mouth of the jar; if mixed with oxygen or atmospheric air, it will explode.

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Hold over the jet of hydrogen issuing from a small tube, hollow cylinders of gla.s.s or earthenware, Florence flasks, or hollow gla.s.s b.a.l.l.s, and musical sounds will be produced, which were supposed to depend on some peculiar property of hydrogen gas, until Mr. Faraday tried flame from coal gas, olefiant gas, and even the vapour of ether, when the sounds were still produced, and he attributed them to a continuous explosion, or series of explosions, produced by the union of oxygen with the hydrogen of the flames.

WATER.

With oxygen, hydrogen unites to form the important compound water, which exists not only in the obvious form of oceans, rivers, lakes, rains, dews, &c. &c. but is found intimately combined with many substances, giving them some of their peculiar properties. Many crystals have a definite proportion of water combined with them, and on losing this water they lose their crystalline form. Many acids also cannot exist as acids without water. The slaking of lime depends upon the union of water with the lime, the dry powder resulting from the process being a _hydrate_ of lime, the water having become _solidified_, and in pa.s.sing from the fluid to the solid state gives out its latent caloric, producing the heat observed during the process. When a large quant.i.ty of lime, a barge-load for instance, has got wetted by accident, the heat evolved has been sufficient to set fire to the barge.

At the temperature of 32 of Fahrenheit's thermometer, water loses its fluid form, and becomes ice. As it solidifies, it starts into beautiful crystals, which unite and cross each other at determinate angles. Ice is lighter than the water on which it floats, forming a protection to the water beneath, and preventing it from being frozen so rapidly; else, if the ice were _heavier_ than water, and consequently sank as soon as formed, each portion of water would be frozen in its turn, until rivers became solid throughout, and every living creature in them must be destroyed. Now, the temperature of the water under the ice is seldom much below 40, and if care be taken to break holes at intervals to allow access to the air, the fish and other aquatic animals seldom suffer even in our coldest winters.

Although it is impossible to raise ice even one degree above 32 without thawing, it is not difficult to reduce water many degrees below that point without freezing it.

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In order to obtain both the const.i.tuents of water in a separate state, it must be decomposed by galvanism, each pole of a battery terminating in a separate tube containing water, when the result will be that at the positive pole oxygen gas will be evolved, and hydrogen at the negative, the latter being double the quant.i.ty of the former. Now, if you mix the gases thus obtained, introduce them into a vessel called a "Eudiometer,"

and pa.s.s an electric spark through them from a Leyden phial, a sudden flash will be seen, and the gases will entirely disappear, being again converted into water. If you have a mercurial trough, and perform this experiment over mercury, the inside of the eudiometer will exhibit minute drops of water. Thus you have proved both by _a.n.a.lysis_ and _synthesis_, that water consists of oxygen and hydrogen, in the proportion of one volume of the former to two of the latter.

EXPERIMENT.

Take some perfectly pure distilled water, filter it, surround it with a mixture of light snow, or powdered ice, and salt, taking care to keep it perfectly still, a thermometer having been previously placed in it. The mercury will gradually sink many degrees below the freezing point 32 (it has been reduced as low as 4), the water still remaining fluid; when all at once, either from shaking the table, or simply because the reduction can be carried no further, it suddenly starts into ice, and the thermometer jumps up at once to 32, where it remains until the whole is frozen, when the temperature gradually sinks to that of the surrounding medium.

Now if you remove the gla.s.s of ice from the freezing mixture into the apartment, and watch the thermometer, you will find it gradually rise to 32, and there remain until all the ice is melted, when it will gradually acquire the temperature of the room. The reason of this is, that the water in pa.s.sing from the solid to the fluid form absorbs, and in pa.s.sing from the fluid to the solid form gives out caloric, so maintaining the temperature at 32, the point at which the change of form takes place, until it is completed.

Between the temperature of 32 and 212, water exists in a fluid form, under ordinary circ.u.mstances; but at the latter point it a.s.sumes the form of vapour or steam, and acquires many of the properties of gases, being indefinitely expansible by heat, the force increasing as the temperature is raised, provided the steam be confined, until it becomes irresistible,--witness the frequent explosions of steam-engines even in this country; and in America, where the engines are worked at a high pressure, accidents are of daily occurrence.

The temperature at which water boils is modified by the pressure applied to it. Thus, as you ascend a mountain, and so pa.s.s through a portion of the atmosphere, water boils at a lower temperature, until at great heights it boils at so low a heat, that good tea cannot be made because it is impossible to heat the water sufficiently. Under the exhausted receiver of an air-pump, water boils at about 140.

CHLORINE.

Another gaseous element, sometimes called a supporter of combustion, is named chlorine, from a Greek word signifying yellowish green.

This gas was formerly called "oxymuriatic acid," being supposed to be a compound of oxygen and muriatic acid gases, until Sir H. Davy, in a series of masterly experiments carried on during the years 1808-9-10 and 11, proved that it contained no oxygen or muriatic acid, and that it was in fact a simple or undecompounded substance, and changed its name to chlorine, which name was, after some discussion, accepted by the scientific world, and is still in use.

This gas may be obtained for experiment, by gently heating in a retort a mixture of muriatic or hydrochloric acid, hydrochloride, as it is now called, with some black oxide of manganese: the muriatic acid, a compound of chlorine and hydrogen, is decomposed, and so is the oxide of manganese, giving out some of its oxygen, which takes the hydrogen from the muriatic acid to form water, while the chlorine gas, with which the hydrogen had been united, is set at liberty, and may be collected in jars over water.

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Chlorine gas is transparent, of a greenish yellow colour, has a peculiar disagreeable taste and smell, and if breathed even in small quant.i.ties, occasions a sensation of suffocation, of tightness in the chest, and violent coughing, attended with great prostration. I have been compelled to retire to bed from having upset a bottle containing some of this gas.

It destroys most vegetable colours when moist, and is in fact the agent now universally employed for bleaching purposes.

It has also the power of combining with and destroying all noxious smells, and is invaluable as a purifier of foul rooms, and destroyer of infection. For these latter purposes it is used in combination with lime, either in substance or solution, under the name of "Chloride of Lime."

Sir W. Burnett has lately discovered that the chloride of zinc answers the same purposes as the chloride of lime, and has the advantage of being itself dest.i.tute of smell, and his fluid is frequently subst.i.tuted for the other.

Chlorine gas is a powerful supporter of combustion, many of the metals taking fire spontaneously when introduced in a fine state of division into the gas.

EXPERIMENTS.

1. Into a jar of chlorine gas introduce a few sheets of copper leaf, sold under the name of Dutch foil, when it will burn with a dull red light.

2. If some metallic antimony in a state of powder be poured into a jar of this gas, it will take fire as it falls, and burn with a bright white light.

3. A small piece of the metal pota.s.sium may be introduced, and will also take fire.

4. A piece of phosphorus will also generally take fire spontaneously when introduced into this gas. In all these cases direct compounds of the substances with chlorine are produced, called chlorides.

5. If a lighted taper be plunged quickly into the gas, it will continue to burn with a dull light, giving off a very large quant.i.ty of smoke, being in fact the carbon of the wax taper, with which the chlorine does not unite; while the other const.i.tuent of the taper, the hydrogen, forms muriatic acid by union with the chlorine.

6. This substance has the property of destroying most vegetable colours, and is used in large quant.i.ties for bleaching calico, linen, and the rags of which paper is made. It is a curious fact that it shows this property only when water is present, for if a piece of coloured cloth is introduced dry into a jar of the gas, also dry, no effect will be produced--wet the cloth, and reintroduce it, and in a very short time its colour will be discharged.

7. Introduce a quant.i.ty of the infusion of the common red cabbage, which is of a beautiful blue colour, into a jar of this gas, and it will instantly become nearly as pale as water, retaining a slight tinge of yellow. A solution of sulphate of indigo can always be obtained, and answers well for this experiment.

MURIATIC ACID GAS, OR HYDRIC CHLORIDE.

With chlorine, hydrogen forms a compound called muriatic, or hydrochloric acid gas. It cannot easily be formed by the direct union of its elements, but is procured from some compound in which it exists ready formed. Common salt (chloride of sodium) is generally employed; and when acted on by strong sulphuric acid (or oil of vitriol), the gas is disengaged in abundance. It must be collected over mercury, for water absorbs it, forming the liquid muriatic, or hydrochloric acid.

A lighted taper plunged into this gas is instantly extinguished. It is very dangerous to animal life if respired. It has the property of destroying animal effluvia, and was once employed to purify the cathedral of Dijon, which was so filled with putrid emanations from the bodies buried in it, that it had been closed for some time. It perfectly succeeded, but it is so destructive to all metallic substances that it is not used now, for the chlorides of lime and zinc have since been discovered to act more effectually than the muriatic acid gas, without its inconvenience.

The compounds of hydrogen with iodine are pa.s.sed over.

With nitrogen, hydrogen unites and forms one of the most extraordinary compounds in the whole range of chemistry,--the gas called ammonia. This is the only gas possessing what are called alkaline properties; _i. e._ it changes the blue colour of certain vegetables to green, yellow to deep brown, and unites with the acids to form neutral compounds, just as the other alkalies, potash and soda, which are oxides of metals. It may be procured in abundance by heating the hydrochlorate of ammonia, or sal ammoniac, as it is usually called, with quick-lime, which takes the hydrochloric acid, and sets free this remarkable gas. It must be received over mercury, as it is absorbed to almost any extent by water, forming the fluid sold as "spirits of hartshorn" in the shops.

This gas is colourless and transparent, lighter than atmospheric air, and will not support combustion; it has a very pungent but not disagreeable smell. Under certain circ.u.mstances it is combustible.

EXPERIMENTS.

1. Take a bottle containing chlorine gas, and invert over its mouth another filled with ammoniacal gas; then if the bottles be held in the hand (guarded by a pair of gloves), and suddenly turned, so that the chlorine be uppermost, the two gases will unite so rapidly that a white flame fills the bottles for an instant.