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Under Optics, the mechanical properties of light were considered.

Light has considerable influence on chemical operations, but little is known of its real nature. Most generally it is considered as a certain simple substance, of which the chief source is the sun; and it is also disengaged during the processes of combustion.

The most delicate experiments have been insti. tuted for the purpose of discovering whether it has weight, but without success; on which account it is reckoned among the imponderable bodies.

There appears to be an intimate connexion between light and heat, and they are frequently given out together. But although they are both always found in the sun's rays, yet from them they may be obtained separately, the invisible

rays of heat being more refrangibible than those of light : see vol. i.

Light is capable of entering in union with many substances, and of being again separated from them. This is the case in the substance called pyrophorus, which is made by exposing to a red heat in a crucible for some time, a mixture of pounded oyster shells and sulphur. If this substance be then carried into the light for a few seconds, it will imbibe so much that it will become luminous in the dark by again giving out this light.

Various kinds of meat, but particularly fish when they are beginning to putrefy, also rotten wood, sea-weeds, and some insects, as the glowworm and lanthorn fly, have the property of shining in the dark.

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The effect of light upon vegetation is well known. Many flowers follow the course of the sun, and most flowers turn themselves more or less towards the light. Plants that grow in darkness are pale and without colour, and when this is the case they are said to be etiolated, or blanched. Gardeners avail themselves of this fact to render some vegetables, as celery and endive, white and tender. The more plants are exposed to the light, the more colour they acquire. Vegetables are not only indebted to light for their colour, but their taste and odour are derived from the same

From this it happens, that hot climates are the native countries of perfumes, odoriferous fruits, and aromatic resins. The action of light on the organs of vegetables causes them to pour out streams of oxygen gas from the surfaces of their leaves, while exposed to the sun, whereas, on the contrary, when in the dark, they emit air of a noxious quality.

Animal life seems also to be no less influenced by light. Birds that inhabit tropical countries have much brighter plumage than those of the north. Animals in general seem to droop when deprived of light; and no doubt it is very essential to the health of human beings.

The colour of metallic oxides is changed by the action of light: the yellow oxide of tungsten becomes blue by exposure to light; the white salts of silver become black, and green precipitate of iron becomes red. Some oxides of metals lose weight by exposure to light, as the red oxide of mercury ; others lose their oxygen entirely, or become reduced, as the oxide of gold. Light, then, has the property of separating oxygen from several of the oxides.

Some substances when heated to a certain degree become luminous; iron, for instance; and this is what is called a red heat.

If bodies heated to redness be introduced into a gas, it does not become visible, and hence it has been concluded that gas is not capable of being made luminous : but it is now considered that flame is hydrogen gas in a luminous state:

Light is also produced by percussion ; as in the case of a flint and steel. The spark produced in this case is owing to the flint breaking off a small fragment of the steel, which is thus rendered red hot, and burns during its passage through the air. But two pieces of quartz struck smartly together also give out light, although here there can be no combustion. : Instruments for measuring the degree or intensity of light are called photometers.



Electricity and galvanism have been already treated of in the first volume. The electric fluid is now considered as a chemical agent of great importance, exciting a powerful influence in the decomposition of bodies. The connexion between electricity and chemical decomposition was first shown by Sir Humphry Davy, to whom the world is indebted for so many brilliant discoveries.

There is still, however, great uncertainty and various opinions with respect to the real nature of this influence, which is usually classed among

the imponderable elementary bodies,


Oxygen is an elementary body that cannot be procured in a separate or free state, that is, it cannot be detached from the other bodies with which it is always combined.

Oxygen gas is so called from two Greek words, signifying the generator of acids, because it was considered by Lavoisier as the only acidifying principle. It has been called also pure or vital air.

About one fourth of the atmosphere consists of this gas, and it is essential to respiration and animal life. It is the most powerful and general supporter of combustion ; and by its union with other bodies,

l it forms most of the acids. Oxygen gas may be easily procured by several processes.

1. It is obtained in the greatest purity from oxy-muriate of potash. Put some of this salt into a small glass retort, place the neck under the shelf of the pneumatic trough, and apply the heat of a lamp to the retort. The salts will soon melt and boil, when oxygen gas will come over in great abundance.

2. Black oxide of manganese is usually employed for furnishing this gas, as it affords it at a cheaper rate. Procure an iron retort made for the purpose, fill it with the oxide, fit a conducting tube to it, and place the retort between the bars of a grate which contains a good fire. Keep up the heat until the retort becomes red hot, and the gas will be received in the pneumatic apparatus. Or it may be made from oxide of manganese, put into a glass retort, with half its weight of strong sulphuric


acid. It be likewise obtained in great quantity from nitrat of potass (salt petre) in an earthen retort exposed to a strong fire; also from the red oxide of lead, heated with or without sulphuric acid.

Having procured a sufficient quantity of this gas in separate vessels, its properties may be easily examined.

It will be found that water does not absorb it ; for if some of it be agitated in a small vial half filled with water, and again immersed into the trough, it will not be diminished in quantity; nor will the water rise in an inverted vessel of this gas, if left on the shelf of the trough for a day.

Oxygen gas is eminently calculated to support the combustion of bodies. Plunge a lighted taper fixed to an iron wire, or a lighted splinter of wood, and the combustion will proceed with a splendour much encreased.

The flame of a lamp urged by a stream of oxygen gas, instead of common air, excites a heat more intense than the hottest furnace.

Even the metals which are not easily combustible in common air burn in oxygen gas with great readi

Iron or steel wire burns in a very striking manner.' It should be kindled by having a small bit of wood fastened to the point; the combustion of this will communicate to the steel wire, which will continue to burn. The fused drops of iron that fall down, when examined, will be found to be no longer malleable, but brittle and converted into the oxide of iron. The same change will take place when the other metals are burnt in this gas.

If a piece of charcoal, fixed to an iron wire, be lighted by a blow pipe, and put into a jar of oxygen gas, it will burn with a brilliant light, and throw



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