THE acid radicals or bases are divisible into two classes, Nonmetallic and Metallic.

All the inorganic acids are connected with this group, with the exception of those of manganese, iron, and osmium; but several of the latter have but slight claims to the characters of acids in consequence of their weak affinities for bases.

HYDROGEN.

History (Mayow, 1674). Inflammable air (Boyle, Hales, and Cavendish, before 1766) Phlogiston. Hydrogen, from idap, water, and yvoμas, I produce (Lavoisier, 1787).

Source. Water. Vegetable and animal substances.

Physical properties.-A transparent, colourless, elastic gas, possessing the mechanical properties of common air, without taste or smell when pure. Spec. grav. 0-0694 (Thomson), or 16 times lighter than oxygen. The weight of 100 cubic inches is 2.153 grains, or about 14 times lighter than air. Refracting power 47, that of air being 1 (Dulong).

Preparation. The source of hydrogen, in whatever manner it may be prepared, is water, which is composed, as Watt suggested,

from Warltire and Priestley's experiments (1781), and as Cavendish demonstrated (1781), of oxygen and hydrogen.

This is the most convenient If we introduce 2 parts of zinc,

Water.

1.125
HO

1. By zinc and sulphuric acid. method of preparing the gas. 3 parts of sulphuric acid, and 16 of water into a retort or flask (figs. 15. and 19.), supplied with a bent tube, heat is produced by the union of sulphuric acid with water, and a copious evolution of hydrogen gas takes place. The action in this case is as re- Hydrogen presented in the diagram.

•125 H

Iron or other metals may be substituted for the zinc.

Zinc.

4.125

Zn

2. By passing steam over red-hot iron.- - An iron, or porcelain tube or gun-barrel, partially filled with iron-filings is to be passed

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through a furnace, and surrounded with red-hot coals; to one extremity is attached, by a cork, a retort containing

water, and to the other also, by means of a cork, a bent tube dipping under

water in a pneumatic trough, (fig. 34.); heat being applied to the water by means of a chauffer, the steam traverses the red-not

tube containing the ironfilings, the oxygen is absorbed by the iron-filings, an oxide of iron being formed (Fe, 03+ FeO or Fe, 04), while the hydrogen passes by the bent tube and may be collected. This experiment is explained by the diagram.

This process affords a plentiful supply of hydrogen at a cheap rate. It is necessary in this experiment to ensure the tightness of the apparatus, otherwise, if common air enter, its oxygen will unite with the hydrogen, and cause a violent explosion. Hydrogen is thus obtained by a process of oxidation. If the process were reversed, and the hydrogen passed over the hot oxide of iron, water would be formed by a process of deoxidation.

3. Hydrogen may be obtained from the decomposition of

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water by the galvanic battery (fig. 35.). An apparatus is usually sold for this purpose, consisting of a glass vessel, in which are deposited two inverted tubes, which are filled with a weak solution of common salt in water, and which are connected with two platinum wires that communicate with the two poles of a battery. Oxygen passes to the positive pole, and twice as much hydrogen is disengaged at the negative pole. Hydrogen is also extricated when potassium or sodium are brought in contact with water; the oxygen uniting with the metal, and forming a soluble oxide, termed caustic potash or soda. Hydrogen, when perfectly pure, should have no smell; but when prepared from zinc or iron it possesses a peculiar odour due to the impurities present in these substances. Arsenic, carbon, or bituminous matter and tin, are generally

Chemical characters.

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present in minute quantities in zine; and it is also suspected that when made by means of sulphuric acid and zinc, a portion of the metal is carried over in a gaseous state in union with the hydrogen. To purify it from these substances, and principally from carbonic acid and bituminous oils which are carried over, it has been recommended to pass it through alcohol (Prout), or caustic potash, and also to wash it with a solution of corrosive sublimate. Experiments. 1. To show that hydrogen is a combustible substance, and not a supporter of combustion, we fill a jar with the gas, and plunge a lighted taper into it, which is fixed to a cork accurately adapted to the mouth of the jar. When the taper is immersed in the gas, and the cork fixed, the combustion terminates. 2. If we introduce the gas into a jar supplied with a narrow tube or gas burner and stop-cock, we can burn the gas by turning the cock, igniting, and gently pressing down the jar in the water trough, and, if the flame is introduced into a dry flask, water will be deposited (fig. 36.). 3. An open jar filled with it may be ignited. In all these cases we observe that the gas burns with a pale yellow flame. 4. To exhibit its lightness we fill a bladder, which is supplied with a stop-cock, with common air. We adjust a common tobacco pipe to it, and blow soap bubbles by

E

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means of a solution of soap, and gentle pressure on the bladder (fig. 37.). These bubbles not being lighter than the air speedily fall to the ground. If hydrogen be now substituted for the common air in the bladder, it will be found that the soap bubbles blown with it will ascend in the atmosphere in consequence of their lightness, and if a light be brought in contact with them they will explode. It was this property of hydrogen that led to aerostation, or the contrivance of raising balloons in the air. Dr. Black seems to have been the first individual who suggested the employment of it for this purpose. Previous to this observation fire balloons were alone used. Dr. Charles caused the first hydrogen balloon to ascend from Paris on the 27th of August, 1783. Common street gas is now employed for this purpose, as it is only about half as heavy as common air. Balloons are frequently formed of taffety, and covered with a linseed oil varnish, glue, and oil of turpentine; varnishes of copal and caoutchouc have also been used. To imitate these, small balloons are sold, which are formed of the allantois of the calf, are exceedingly light, but are very frequently not air tight. 5. When a small flask is supplied with a perforated cork and tube terminating in a capillary opening, and hydrogen is disengaged within it from zinc and sulphuric acid, the gas escapes by the capillary tube, and can be ignited. If a long wide tube be carefully introduced, so as to surround the lighted gas, musical notes will be produced, varying in depth according to the length of the tubes; a variety of tubes will produce different notes. This phenomenon is produced by a series of small explosions, which follow each other so rapidly that they produce a continuous sound (Higgins, 1777). A similar effect takes place when carbonic oxide is burned. If a common medicine phial be filled with hydrogen, and common air, inverted and ignited, an identical sound is perceptible. 6. The heating power of hydrogen may be exhibited by taking a flask supplied with a rectangular tube, and placing under its extreme end which should be drawn out to a fine capillary point,- a piece of spongy platinum on a stand (fig. 39.). By evolving hydrogen in the flask, and causing it to project upon the platinum, so much heat is evolved as to make the metal red-hot, and to inflame the hydrogen, thus rendering this simple apparatus capable of producing an instantaneous light. Various modifications of this principle have been devised, as in Garden's lamp, consisting of two globular glass vessels, and a tube and stop-cock for the issue of the gas connected with the inferior vessel (fig. 38.). The lower vessel being nearly filled with water and dilute sulphuric acid, the upper one is inserted in it

empty, having a piece of zinc attached to the lower end of the tube. The hydrogen begins immediately to be disengaged, and, pressing

on the surface of the fluid, forces it up the tube into the upper vessel, when the zinc being freed from the sulphuric acid, the action ceases. The lamp is now ready for action. The stop-cock is turned, and the hydrogen, in consequence of the pressure of the column of water, is forced along the tube, and striking upon the spongy platinum placed in the little holder, causes it to become red-hot, as in the previous experiment. Fig. 40., a German lamp.

The explanation which has been given of the cause of this great evolution of heat is as follows: hammered platinum has a specific gravity of about 22, water being 1, while that of spongy platinum is only 16. Hence it appears that in every cubic inch of spongy platinum of a cubic inch is not occupied by metal, but is distributed through the powder in the form of space or a kind of capillary tubes. Now it has been ascertained that 1 cubic inch of spongy platinum absorbs 253 cubic inches of oxygen, which must therefore be compressed into the space of a quarter of a cubic inch, which is about a thousand times smaller bulk than the gas occupies at the usual temperature and pressure of the atmosphere. This condensation must, therefore, bring the gas nearly to the condition of a fluid, and enable hydrogen to enter into closer contact and unite with it. The change of the two gases in uniting to form water-from a rarer to a denser statecauses the evolution of all the heat required to keep them in the condition of gases; the platinum therefore becomes red-hot. It has been found, however, that it is not necessary for the platinum to be in a finely divided state; for if a clean plate of platinum be introduced into a mixture of two volumes of hydrogen and one of oxygen, union takes place and water is formed, the two gases being condensed it is supposed on the surface of the platinum either by an electrical or contact action (Faraday), to which the term catalytic has been unnecessarily applied.

7. The heating and illuminating power of hydrogen, when con