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The authors entertain views rather different from those which generally prevail regarding the relative importance of the various parts of chemistry; they have endeavoured to make the teaching given in this book sound so far as it goes; they have tried to bind together the facts and principles of the science, and at the same time to avoid speculation.

M. M. PATTISON MUIR.

CHARLES SLATER.

CAMBRIDGE, October, 1887.

CORRECTIONS AND ADDITIONS.

PAR

PAGE.
222

317

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to table add MONATOMIC ATOMS Na, K.
to list of monovalent atoms add Rb, Cs, Ag.
divalent

add Mn.
trivalent

add Al, Cr.
to list of gaseous molecules on which classification of
atoms is based add, RbCI, RbI, CsCl, CsI, AgCl; PbCl2,
MnCl2 ; AlCl3, CrCiz.

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ELEMENTARY CHEMISTRY.

CHAPTER I.

CHEMICAL CHANGE.

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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.

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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

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