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CHEMISTRY is a branch of natural science. The aim of 1 science is to "see things as they are.” But to see things as they are it is necessary to study the relations of things, because in nature nothing is wholly cut off from other things, but everything is either a cause or a consequence of many others, and is related in manifold ways even to things which may seem to be wholly unconnected with it.
For the purposes of exact study however some boundary lines must be drawn between what we call the different parts of each natural occurrence. Every natural occurrence, in relation to our powers of comprehending it, is infinitely complex; in order to explain we must simplify; and to simplify we must overlook some portions of the complete phenomenon,
Chemistry deals with certain portions of one class of material phenomena. The mark of this class of phenomena is, change of properties accompanying change of composition. The object of chemistry is to classify the phenomena it studies in order to discover general laws.
The object of this book is to place before the student an outline of the methods by which chemistry proceeds; to teach him some of the general laws of the science; and above all things to shew him that the laws are gained by studying natural occurrences, that the detailed study of these is the foundation on which the science rests, but that, in so far as it is a real branch of science, chemistry is much more than a descriptive catalogue of interesting facts. M. E. C.
A little observation suffices to shew that all things are undergoing change. Physics and chemistry deal with the phenomena presented in material changes. Certain aspects of these changes we call physical ; certain aspects of them we call chemical.
A fire burns on the hearth: when the fire was kindled the grate was filled with lumps of coal; as the fire sparkles and blazes up the black coal changes to a light-giving, glowing, mass, radiating heat on all sides; as the flames cease to play about the glowing coals the colour fades, the ashes accumulate, and the burning slackens; at last the change stops, there remain only ashes and some pieces of unburnt coal.
Many of the changes which pass before us as we watch the progress of a coal-fire are chemical changes. It is with such processes as this that we are concerned.
The burning and slow extinction of an ordinary fire is however an extremely complex event; we must turn to comparatively simple occurrences if we are to learn the characteristic features of chemical change.
When a piece of platinum wire is held in the flame of a Bunsen-lamp it becomes hot and gives out light; when the wire is removed from the source of heat it quickly cools, ceases to emit light, and returns to the same condition as before heating. When a piece of magnesium wire or ribbon is brought into the lamp-flame it also becomes hot and gives out light, but at the same time it rapidly burns away ; when removed from the source of heat it continues to burn with emission of dazzling white light; after a little the burning ceases; the magnesium has now disappeared and in its place there is formed a white, soft, powder, called magnesia, very unlike the hard, lustrous, magnesium which was placed in the lamp-flame.
Some change was here produced in the properties both of the platinum and magnesium. In the case of the platinum, the properties of glowing and of communicating heat to colder bodies brought into contact with it were temporarily added to the other properties-hardness, lustre, tenacity, high specific gravity, infusibility, &c.—which distinguish platinum from other kinds of matter. When those properties which had been temporarily added were withdrawn, the platinum was found to exhibit the same properties which characterised it before it was brought into the lamp-flame. In the second case, the magnesium also temporarily acquired the properties of glowing
and communicating heat to colder bodies brought into contact with it. But the withdrawal of these properties did not leave the magnesium as it was before heating; accompanying the exhibition of these properties there was a gradual change of the magnesium into a substance so unlike magnesium as to be at once recognised as a different kind of matter.
If a thin copper wire, covered with cotton or silk, is 3 wrapped many times round a piece of soft-iron and an electric current is then passed through the wire, the iron will acquire the property of attracting iron-filings; if the electric current is stopped the iron-filings cease to be attracted ; if the current is again passed through the wire the soft-iron at once acquires the attractive power. Before, during, and after, the passage of the electric current, the soft-iron exhibits all those properties—colour, relative density, tenacity, malleability, texture, &c.—which mark it off from other kinds of matter; while the current is passing there is added to the iron the attracting iron-filings. If the same piece of soft-iron is exposed to damp air for a considerable time, a portion of it, or if sufficient time be given the whole of it, will be changed into iron-rust, which is a reddish powder unlike the iron in texture, colour, lustre, tenacity, malleability, and many other properties.
The change of iron to iron-rust resembles the change of magnesium to magnesia, in that in both cases there is produced a new kind of matter. The temporary addition to iron of the property of attracting iron-filings resembles the temporary addition to platinum of the property of glowing and communicating heat to colder bodies, in that in both cases the change consists in the addition of a property which does not destroy or mask the original properties, and which can be withdrawn by reverting to the conditions existent before this property was added.
Let a rod of copper and an electric bell be introduced into 4 the circuit of a galvanic battery (s. fig. 1); the ringing of the bell shews that the electric current is passing through the rod
The moment the current is broken the bell ceases to ring. The characteristic properties of the copper are not modified by the passage through it of the electric current. Let the rod of copper be now removed; let a piece of sheetplatinum be attached to the end of the wire from the battery, and also to the end of the wire from the electric bell, and let these pieces of platinum be placed, side by side but not touching, in a dilute aqueous solution of copper sulphate to
Fig. 1. which some sulphuric acid has been added (s. fig. 2). The ringing of the bell shews that the electric current is passing through the solution of copper sulphate; but the passage of the current is accompanied by the deposition on one of the
platinum plates of a reddish solid which may be proved to be copper, and by the gradual disappearance of the copper sulphate from the solution. When the current is stopped there remains the new kind of matter, copper, which has been formed by the action of the electric current on the kind of matter originally present, copper sulphate; a certain amount of one kind of matter has disappeared and a certain amount of another kind of matter has been formed. The original matter is not reproduced by reverting to the conditions which existed before the change began; that is to say, by stopping the passage of the electric current. The three kinds of matter, platinum, soft-iron, and copper, ,
5 have been changed by temporarily adding to each a property which does not belong to it under ordinary conditions ; this property existed only so long as the special conditions which caused its existence continued; the withdrawal of these conditions was accompanied by the withdrawal of the special property; when this property was withdrawn the platinum, soft-iron, or copper, remained exactly as it was before the change had been effected. On the other hand, the three kinds of matter, magnesium, iron, and copper sulphate, have been changed by each permanently losing certain properties which characterise it, and at the same time permanently acquiring new properties which characterise other kinds of matter.
Now we recognise different substances by their properties. 6 One kind of matter is recognised, and distinguished from other kinds of matter, by its colour, texture, brittleness, opacity, relative density, hardness, &c.; also by its behaviour when heated, when submitted to the action of electricity, placed in direct sunlight, mixed with water, brought into contact with other substances at high and low temperatures, &c. Substances which have markedly different properties are said to differ in kind, or to be different kinds of matter. Thus we say that iron is a kind of matter different from glass; that sand is a kind of matter different from wood, &c.
The prominent feature of the change undergone by the 7 magnesium when heated, by the iron when allowed to remain in damp air, and by the copper sulphate when the electric current was passed through it, is, that in each case a kind of matter has been produced different from, and in the place of, that which existed before the change began. The prominent feature of the change undergone by the platinum when heated, by the iron when the current was passed round it, and by the