sulphur and the compound saltpetre. The compound saltpetre contains the elements potassium, nitrogen, and oxygen.

2. The changes which they undergo by the action of other bodies and of the various forces upon them.-Phosphorus, which is obtained from bones, is luminous in the air, and is gradually converted into an acid liquid; it unites with the oxygen of the air and with water, as the iron did when exposed to moist air. If the phosphorus is gently heated, this union is attended with a vivid combustion, and there is formed a body which is different from the former; to which, if water and lime be added, a new body is formed, very similar to boneashes; it is, in fact, artificial bone-ashes. The number of new bodies which may be produced by the union of the elements with each other, or with compound bodies, is infinite; and entirely different substances are often formed, according as the combination takes place under the influence of cold or heat, in water or in air, in greater or smaller quantities. This is combination or synthesis.

3. The causes of chemical changes and the laws according to which they take place. If chemical experiments are performed as they should be, with the balance in the hand, it will soon be observed, that when two different bodies which can unite with each other are brought together, sometimes a part of the one, sometimes a part of the other, remains free. Further experiments will show how much of one body, in weight, can be united with the other. If all bodies are tested in the same manner, the certainty is finally attained, that all chemical combinations take place only in fixed, unchangeable proportions, and that to every individual body is assigned a definite weight, with which it always enters into any combination whatever. (Chap. III.) This certainty is called a natural law. Many such laws of nature have already been ascertained, and they serve as a certain guide to the chemist in his labours, since they cannot, like human laws, be arbitrarily evaded or changed. By them alone we attain to a scientific insight into chemical processes, and to the capability of putting direct questions to bodies by experiment, and of testing the truth of the answers received. An explanation of chemical processes based on natural laws, which presents a clear idea of the subject to the mind, is called a Theory.

4. The extent to which the facts which have been discovered

may be made useful to man.—' -When the chemist discovers a new body, or a new property in one already known, or a new method of synthesis or analysis, he imparts his discovery to the apothecary, the physician, the farmer, the manufacturer, and the tradesman, that experiments may be instituted for the purpose of ascertaining whether any advantage, facility, or improvement can be derived for pharmacy, medicine, agriculture, or the arts. Phosphorus ignites spontaneously at a gentle heat; it is used in lucifer matches. Taken into the stomach, it acts as a yiolent poison; it is at present the most common means for the destruction of rats and mice. The constituents of bone-earth and those of gelatine have been found to be universally present in the seeds of different kinds of corn; the chemist concludes from this that pulverised bones should yield an excellent manure for corn; the agriculturist demonstrates this by experiments on a large scale. In bone-black the property has been discovered of attracting many substances held in solution in liquids, and of condensing them in itself: on account of this property it is used for making impure water potable; the sugar-refiner employs it to make brown syrup colourless; with it the distiller purifies spirit from fusel oil. This is applied or practical chemistry.

Nothing is better calculated to excite an interest in chemical knowledge than a consideration of the useful application which can be made of it in every-day life. Chemistry teaches the apothecary how to compound and prepare his medicines; it teaches the physician how to cure maladies by means of these medicines; it not only shows the miner the metals concealed in rocks, but aids him also in smelting and working them. Chemistry, in connection with physics, has been the principal lever by which so many arts and trades have been brought to such a degree of perfection within the last few decades, and by its means we have been supplied with numberless conveniences of life that were not enjoyed by our fathers. It cannot be doubted that the farmer must at once regard chemistry as his indispensable friend, for it is this alone which acquaints him with the constituent parts of his soil, with the proper nutriment of the plants he wishes to cultivate, and with the means whereby he can enhance the fruitfulness of his fields.

The ancient so-called Elements.-The dogma of the ancient philosophers, that there are four elements, earth, air, water, and fire, has long been known to be erroneous, although the words are still repeated very often. We now know that three of them are either mixtures or compounds of the true elements, and that fire is but a peculiar operation of heat and light.

CHAPTER II.

GRAVITY, HEAT, PRESSURE.

GRAVITY.

THE force of gravity is chiefly interesting to chemists because it confers upon all kinds of matter the property called weight.

WEIGHING AND MEASURING.

Weighing.-The balance is to the chemist what the compass is to the mariner. The ocean was indeed navigated before the discovery of the compass; but not till after this could the sailor steer with confidence to a certain place, and recover his proper course, however often lost. And so, in chemistry, no systematic method of study could be pursued before the introduction of the balance. The balance is the plumb-line, as well as the touchstone in chemical experiments; it teaches us how to ascertain the true composition of bodies, and shows us whether the questions put, the answers received, or the conclusions drawn from them, are correct or false. Hence it cannot be too strongly recommended to those commencing the study of chemistry to use the balance even in simple experiments. For the experiments described in this book, a common apothecaries' balance is all that is requisite.

Such a balance consists of a brass or steel lever (beam), with arms of equal length, through the centre of which passes a steel wedge-shaped axis, resting on a hardened plate, so that the beam, to the extremities of which the pans are attached, may easily vibrate. It is essential that the axis should be in the right place in the beam, a little above its centre of gravity, as in Fig. 2, a. The centre of gravity can be found by balancing the beam on its flat side, with the

index attached to it, upon a knitting-needle, and when the beam rests horizontally, the point of the needle designates the

Fig. 2.

α.

centre of gravity. If the axis be placed too low beneath the centre of gravity, as in Fig. 2, b, the beam will overset, if one of the pans is more heavily loaded than the other. If placed directly in the centre of gravity, the balance itself will cease to vibrate when the beam is in an oblique position. When the axis is too high above the centre of gravity, the balance loses much of its sensibility. This latter defect occurs most frequently, but is easily remedied by lowering the axis. Avoirdupois weight is most frequently used in England. The only numbers required in chemistry are the following:

Apothecaries' weight is somewhat different, but the grain

The new French system of weights and measures, which is now almost universally adopted by chemists, is characterised by great simplicity, all its divisions being made by ten; hence the name decimal weight and measure. Its unit is derived from the size of our globe.

In order to define the different localities on this globe, imaginary circles, as is well known, have been drawn around it. Those which pass round the earth from east to west, the largest of which is the equator, are called parallels of latitude (circles of latitude); those which pass round the earth lengthwise, intersecting at the poles, meridians (circles of longitude). The parallels of latitude gradually become