from the experiments of Sir H. Davy, to contain about fifty-seven parts in the hundred of oxygen, to forty-three of nitrogen.

IV. HYDROGEN, as its name imports, is the basis or constituent of water, being derived from the Greek hydor, water. Because it is the constituent of water, however, it does not follow that this ingredient constitutes the largest proportion: for, in fact, water contains eighty-five parts of oxygen to fifteen of hydrogen.

Like oxygen, hydrogen is never found but in a state of combination; the nearest approach to a state of purity is that in which it is combined only with caloric, or in the form of hydrogen gas.

This gas was formerly known by the name of inflammable air, from its property of easily catching fire. From its great degree of levity, being twelve times lighter than common air, it has been employed for the inflating of air balloons.

Being a constituent part of water, whatever process decomposes water will produce hydrogen gas, provided the oxygen is absorbed by any matter, or otherwise disposed of. Thus, if water is dropped gradually through a gun-barrel made red hot in the middle, the water will be decomposed; the oxygen will form an oxide or rust with the iron, and the hydrogen will come out pure by a tube at the other end. The same effect is produced as often as a smith plunges a red hot iron in water. The hydrogen rises with

the vapour, and is sensible to the smell, and the scales which fall off are oxide of iron. But the usual method of obtaining hydrogen gas is by pouring sulphuric acid diluted with water upon zinc or iron filings. In this case the metal is converted into an oxide, or rather a salt (sulphat of iron or zinc), which contains a superabundance of oxygen. Thus the watery fluid is decomposed, the oxygen entering into the composition of the salt, while the hydrogen flies off in the form of hydrogen gas. The same apparatus will answer for this experiment as that described above for the obtaining of oxygen gas.

If a phial is filled with this gas, and a candle is brought to its mouth, it will burn, as it rises by its levity out of the phial, till the whole is consumed. It is incapable, however, of supporting flame or combustion of itself; for if a lighted taper is plunged into a jar of hydrogen gas, it will immediately be extinguished. These phænomena admit of an easy explanation on the principles already established. Hydrogen gas burns like other combustibles by its strong attraction for oxygen, with which it forms water and both gasses being condensed, the caloric which kept them in the gaseous state is set free, and becomes sensible in the form of flame. The air of our atmosphere, you have seen, contains about twenty-three parts in a hundred of oxygen gas, and while there is a fresh supply of it the hydrogen gas will continue to burn. Hydrogen

VOL. II.

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gas, on the contrary, will not of itself support combustion, because the presence of oxygen gas is absolutely necessary to this effect, which is in this case the result of the union of these two gasses.

Hydrogen combines with the other simple combustibles, forming sulphuretted hydrogen gas, phosphuretted hydrogen gas, carburetted hydrogen gas, &c,

The latter gas, obtained by the combustion of coal, is that which is employed for the purposes of illumination. It is this species of gas which forms the fire-damp or blower, so destructive by its explosion, on the contact with flame in coal mines. Against this evil, Sir H. Davy has opposed an excellent contrivance, the safetylamp, in which the flame of the lamp, being surrounded with wire gauze, the communication of flame to the external explosive gas is prevented by the cooling power of the metallic tissue.

Hydrogen gas is not so pernicious when respired as some other of the gasses. If much diluted with common air, indeed, it may be breathed without injury.

V. PHOSPHORUS is one of those simple acidifiable and inflammable substances which we find in a solid form, though I do not know that it is any where found native; all that is used in philo sophy and commerce being obtained by an artificial process. As in the state of an acid it forms one of the constituent parts of bones,

which are chiefly composed of phosphat of lime, it is easily driven off from burnt bones by a stronger acid. But the mode of obtaining it, and its most conspicuous properties, will be detailed more at large when we treat of combustible bodies. It is only necessary at present to mention it as a simple substance, and to notice its combinations.

Phosphorus, when pure in colour and substance, resembles bees-wax. Its most remarkable property is its very strong attraction for oxygen, from which circumstance it burns spontaneously in the open air at the temperature of 43°. That is, in philosophical language, it attracts the oxygen gas from the atmosphere, which becoming condensed, heat and flame are produced. The consequence is, that the phosphorus becomes oxidized or converted into an acid. Phosphoric acid is heavier than the phosphorus which produced it in proportion to the quantity of oxygen which is combined with it.

Phosphorus combines with the other simple combustibles, as well as with many other bodies forming phosphurets.

in a

solid

VI. SULPHUR is also one of those simple substances which is known to exist pure state. It is familiar to us under the old homely name of brimstone, and therefore requires no description. It is found combined with many mineral substances, as arsenic, antimony, copper, and most of the metallic ores, from which (being very volatile) it is easily driven off by

heat, and collected in proper vessels. But I shall have to introduce more observations upon it under the class of Combustibles, and therefore have now only to remark its combinations.

Like phosphorus, it has a strong attraction for oxygen, and therefore is easily converted into an acid by combustion in the open air. A small dose of oxygen produces sulphurous acid, a larger portion sulphuric acid. The sulphuric acid (formerly called the vitriolic) is a very powerful agent in chemistry. But of its properties it will be necessary to treat under the class of acids.

Sulphur unites with the other simple combustibles, and some other bodies, and forms sulphurets.

VII. CARBON is a substance very plentifully diffused throughout nature, since it enters into the composition of all vegetable and animal bodies. It is, however, only found pure and in a solid form in that most estimable of gems the diamond, which by various experiments is found to be pure carbon in a crystallized state. It is, however, found sufficiently pure for chemical purposes in the charcoal of wood, and will be treated of at large under the class of combustibles. I have at present only briefly to notice its combinations.

With oxygen, carbon forms carbonic acid, which is found naturally united with several substances. It exists in large quantities, united with chalk, limestone, marbles, &c. and in most.