Images de page





PERSONS in general look at the magnificent fabric of civilized society as the result of the accumulated labour, ingenuity, and enterprize of man, through a long course of ages, without attempting to define what has been owing to the different branches of human industry and science; and usually attribute to politicians, statesmen, and warriors, a much greater share than really belongs to them in the work: while what they have done is in reality little. The beginning of civilization is the discovery of some useful arts by which men acquire property, comforts, or luxuries. The necessity or desire of preserving them leads to laws and social institutions. The discovery of peculiar arts gives superiority, to subjugate other nations, who learn their arts, and ultimately adopt their manners; so that, in reality, the origin, as well as the progress and improvement of civil society, is founded in mechanical and chemical inventions. No people have


ever arrived at any degree of perfection in their institutions, who have not possessed, in a high degree, the useful and refined arts. The comparison of savage and civilized man, in fact, demonstrates the triumph of chemical and mechanical philosophy, as the causes not only of the physical, but ultimately even of moral improvement Look at the condition of man in the lowest state in which we are acquainted with him. Take the native of New Holland, advanced only a few steps above the animal creation, and that principally by the use of fire; naked, defending himself against wild beasts, or killing them for food, only by weapons made of wood hardened in the fire, or pointed with stones or fish bones; living only in holes dug out of the earth, or in huts rudely constructed of a few branches of trees covered with grass; having no approach to the enjoyment of luxuries, or even comforts; unable to provide for his most pressing wants; having a language scarcely articulate, relating only to the great objects of nature, or to his most pressing necessities or desires; and living solitary or in single families, unacquainted with religion, government, or laws, and submitted to the mercy of nature or the elements. How different is man in his highest state of cultivation! every part of his body covered with the products of different chemical and mechanical arts, made not only useful in protecting him from the inclemency of the seasons, but combined in forms of beauty and variety; creating out of the dust of the earth, from the clay under his feet, instruments of use and ornament; extracting metals from the rude ore, and giving to them a hundred different shapes for a thousand different purposes; selecting and improving the vegetable productions with which he covers the earth; not only subduing, but taming and domesticating the wildest, the fleetest, and the strongest inhabitants of the wood, the mountain, and the air; making the winds carry him on every part of the immense ocean, and compelling the elements of air, water, and even fire, as it were, to

labour for him; concentrating, in small space, materials which act as the thunderbolt, and directing their energies so as to destroy at immense distances; blasting the rock, removing the mountain, carrying water from the valley to the hill; perpetuating thought in imperishable words, rendering immortal the exertion of genius, and presenting them as common property to all awakening minds; becoming, as it were, the true image of Divine intelligence, receiving and bestowing the breath of life in the influence of civilization.-Sir Humphry Davy.



WHEN a heavy body is raised by a lever, when a stone is raised by pulleys, or a piece of wood split by a wedge, the nature of the substance operated on still remains the same; its position or shape only is altered. These, then, are mechanical actions. The wedge is a mechanical instrument, or power, for facilitating labour. But, when wood is exposed to a strong heat in an iron vessel almost closed, it is converted into charcoal, tar, vinegar, and a quantity of gases. Here there is no change in the position or form of the mass of wood, for the charcoal which remains is in the same situation, and of the same shape, and nearly of the same size as the piece of wood we used; but it is much lighter, for a great many of its particles have left it, to be turned into tar or vinegar: it is now black, whereas, before it was of a whitish yellow colour; it is now brittle, whereas, before it was tough, and could not be easily broken; if set fire to now it will not burn with a bright yellow flame, as it would have done formerly, but with a dull red light.

This, then, is a striking example of the difference between chemical and mechanical action.

When we

applied mechanical action by the wedge, we split the wood into pieces, differing from the original piece of wood, and from each other, in shape and size only, but exactly the same in structure and composition. But when the wood was burnt, by which a chemical action was induced, the wood was decomposed, or divided into several kinds of substance,-charcoal, tar, vinegar, and gas; all differing much in their nature and properties from the original piece of wood, and from each other.

In burning coal in an open fire, the formation of the smoke and ashes out of a piece of solid coal is a chemical action; but the ascent of the smoke in the air-its change of position-is dependent on mechanical principles.

When we raise water by means of a pump, (let us suppose the water contains iron,) the piston and rod are of the same shape and composition during every part of the process, but they have frequently changed position; the water is made to change its situation, but it is of the same nature and composition when brought to the surface, as when ten or twenty feet under ground; above, it contains the iron as well as below. But let us apply heat, and distill the water, we then separate the particles of pure water from the particles of iron with which they are associated,—the water rises in vapour, the iron remains behind. Now, raising the water by the pump is a mechanical process or change; separating the iron from the pure water is a chemical process or change.

Mechanical action, then, is attended with sensible (apparent) motion, or change of position or of form. Chemical action is not necessarily accompanied with sensible or apparent motion, but is attended by a change in the nature and properties of the substances on which we operate. Chemistry, then, effects changes in the nature and properties of bodies, as exemplified above in the cases of soap and glass. By the study of chemistry, we learn the composition of bodies, as, that oil and soda enter into the composition of soap; and we learn how

substances are made to acquire that great variety of properties by which we can apply them to so many different purposes.



WHEN We look abroad on the world around us, one of the first things that strikes us, is the variety in the objects that present themselves to our notice. The endless variety of forms and appearances which animals, vegetables, and other kinds of bodies present, would seem to render it almost a hopeless task to acquire a knowledge of the different substances of which they are composed; but it fortunately happens, that all the diversified works of nature are made up of a few simple materials; that we do not need, in studying every different kind of animal or vegetable, every different air or liquid, every kind of rock or mineral, to learn the properties of a new and different substance for each. There are many thousand kinds of stones and minerals, and other inanimate substances; there is no end to the variety of animals; there are sixty thousand different kinds of plants known, but, however different they may be from each other in structure or appearance, they are made up of combinations, as they are called, of a few simple substances, not above fifty-four in number, of which chemistry has discovered this endless variety to be composed. It may seem surprising, that objects so very opposite to each other, in every respect, should be composed of similar materials; but, if we reflect on the varied forms of building which man can construct out of a few bricks, or on the infinite variety of words, even in one language, which are made up of the letters of the alphabet, we shall not wonder at the varied forms into

« PrécédentContinuer »