EXPERIMENTAL CHEMISTRY.

PART I.

GENERAL PRINCIPLES.

CHAPTER I.

MATTER AND FORCE.

PUTTING on one side as unsolved, and probably for ever insoluble, the great metaphysical question whether the various objects and the complex phenomena we observe in nature have any real existence, or are merely images created in our own consciousness; and the farther and scarcely less recondite question whether, even if we admit the external existence of such objects and phenomena, we may not account for them without assuming the existence of any material substance; let us take the ordinary, and what may be called the common-sense view of the universe around us. In doing so, however, we must not venture to assert it dogmatically, or to stigmatise as absurd either of the other views above referred to, startling though they may appear. Both of them have been and are held by many profound thinkers; neither can be proved to be erroneous, neither can be said to be in direct discord with known facts. The thoughtful student will find it good to revert to them from time to time as he advances in knowledge, and as he gains a clearer conception of their meaning he will find them less and less incredible.

Taking then the current view, we hold that all substances known to us, whether in the state of solid, liquid, or gas,

B

consist of matter which has an absolute existence. Matter is subject to the operation of certain agents called forces, which determine its position in space, and confer upon it its properties. The forces can only be known to us by their operations on matter, just as motion cannot be conceived of except as of something moved. On the other hand, it is no less true that, apart from force, we can form no conception of matter.

Different Forms of Force.-The following are the chief kinds of force.

1. Motion. However motion may have been produced, it is, as long as it remains, a true force. A stone moving through the air is exerting active force, and the planets exert a similar force in their revolution round the sun.

2. Gravity. If a stone is suspended by a string, every one knows that the string is pulled by it. The stone and the earth attract one another, and if the string be cut, the stone will fly to the earth and come to rest. This kind of attraction is called gravitation, and it is said to be due to the force of gravity. It is found to exist in all forms of matter. Solids, liquids, and gases all tend to fly to the earth, and therefore all exhibit the force of gravity, though in very various degrees. Near its surface the great bulk of the earth causes its attraction to be so very much greater than that of any other body that the term gravity is generally applied only to the tendency of bodies to fly to the earth. In other words, gravity is generally taken as identical with weight. But it is right to remember that all bodies attract one another. One stone attracts another, one drop of water another drop; but the masses of the stones and the drops are so small that the attraction is too slight to be noticed. That the force of gravity keeps the planets in their orbits, and regulates the movements of the heavenly bodies, is known to all. It is the most important doctrine of astronomy. When the force of gravity is not resisted by some other force equal in power to itself, motion is produced.

The special case of gravity that we call weight is so im portant in the study of chemistry that it will be considered separately in the next chapter.

3. Cohesion. The force which binds together the minute particles of which matter consists is called cohesion, but it

It varies very

is probably identical in nature with gravity. much in different substances, and even in the same substance in different states. To divide ice into smaller portions requires greater force than to divide water, and no force at all is required to divide steam. In solid bodies cohesion is stronger than in liquids; in gases scarcely a trace of it can be perceived. The hardness of a solid is the measure of the force of cohesion among its particles.

When cohesion is exerted between dissimilar substances it is often called adhesion. The use of glue is a good example of adhesion.

Experiments. To prove that the force of cohesion exists in liquids, allow one pan of a pair of scales to rest on the surface of water. A number of weights must now be introduced into the other pan, before the first one will be removed from the water. When at last it rises, some water will still adhere to its under surface. The adhesion between the scalepan and the water is not overcome, but only the cohesion between one portion of the water and the rest.

If only half the weights required in the above experiment be introduced, and a little ether be then poured on the surface of the water, the scale-pan will rise at once. The adhesion between the ether and the scale-pan is very slight.

Cohesion may also be illustrated by cutting a bullet in half, and pressing together with the hands the two clean and recently cut surfaces. If the cut has been well made, they will unite again with great force. Two pieces of plate-glass will do the same, and the glass-makers have to be very careful not to allow large sheets of plate-glass to lie on one another.

4. Heat is an extremely powerful and important form of force. Its influence in chemistry is very extensive, and, like gravity, it will be considered in some detail in the next chapter.

5. Light is a force closely allied to heat, and is also closely concerned in many of the changes of chemistry. Almost the whole of the light and heat of our globe comes to us directly or indirectly from the sun.

6. Electricity.-Experiment 1.-Take a stick of sealing-wax, rub it briskly with a piece of flannel and hold it near some scraps of paper. They will be attracted by it and will fly

upward and adhere to its surface. A tube of glass rubbed with silk will produce the same effect, but the substance called ebonite is more powerful than either. An ebonite comb that has been passed through dry hair will not only attract scraps of paper and small feathers, but in dry weather will emit a crackling sound and a torrent of small sparks, quite visible in a dark room.

In all these cases the strange and interesting force called Electricity is set in motion. The "electrical machine” is only a plate or cylinder of glass or ebonite, which, by turning a handle, can be made to rub against a silk cushion covered with amalgam. Electricity produced in this way is called frictional electricity.

Another kind of electricity is known, which is called Voltaic, or Galvanic Electricity. The following experiment will illustrate the way in which it is produced.

Experiment 2.-Take a tumbler two-thirds full of water, pour into it gradually an ounce and a half by measure of sulphuric acid (oil of vitriol), and fill the tumbler up with water. The mixture will become very hot, and must be allowed to stand until cold. Very likely a white sediment will form, but this, which is owing to the presence of lead in the acid, will soon subside, and will do no harm. Take a strip of sheet copper, and another of zinc, not too large to go to the bottom of the tumbler, and high enough to reach to the top. Brighten one of the small ends of each with sandpaper, and solder on each bright patch a piece of copper wire about a foot long. It will improve the experiment very much if you can amalgamate the zinc plate; that is, coat it with mercury (quicksilver.) To do this the zinc must be cleaned by immersing it for a minute in the weak sulphuric acid, and then, while covered with the acid, pouring a little mercury on it and rubbing it over the surface with a small rag. The surface of the metal will then become as bright as a looking-glass.

The two metals are now to be immersed in the acid with the wires out. They should be about half an inch apart, and must not touch one another. They can be kept in their places by wedges of cork or wood. If the zinc plate has been amalgamated no action will take place until the ends of the wires are made to touch one another, when the zinc will

immediately begin to dissolve in the acid and electricity to pass along the wires. Such an arrangement constitutes a simple and not very powerful form of voltaic or galvanic battery. It is called a simple cell," and the ends of the wires farthest from the plates are called the "poles." If ten such cells are made, and are joined together by thick copper wires, the zinc of one cell to the copper of the next, long wires proceeding from the last zinc at one end and the last copper at the other, a tolerably powerful battery will be obtained, capable of decomposing water and doing other chemical work, but the arrangement is never a very powerful one, and it is better to buy or make one of the more powerful forms of the instrument.

If the two wires from the simple cell are brought to within half an inch of one another, and are then joined with a piece of very fine platinum wire, the platinum will immediately become red-hot. With a more powerful battery a much greater length of wire may be heated to whiteness, and even melted by the electricity. Fine iron wire may be burned in the same way, but not quite so easily.

Magnetism is only a peculiar operation of electricity.

7. Chemical Force, or Chemical Affinity.-The last force that we have to consider is called chemical force, because the science of chemistry is almost entirely occupied with its nature and effects. Its operations are spoken of as chemical action.

Every one knows that iron, heated to redness, changes into scales or cinders, and that exposed to moist air or earth, it is converted into rust; that the expressed juice of the grape gradually turns to wine, and this again to vinegar; that wood in a stove, or oil in a lamp, disappears in burning; and that animal and vegetable substances in time putrefy, disintegrate, and finally disappear.

Iron cinders and rust are iron altered in constitution; iron is hard, tenacious, of a greyish-white colour and brilliant; by heating to redness it becomes black, dull and brittle; on exposure to moisture it is converted into a powder of a yellowish-brown colour. Wine is altered must, in which nothing of the sweet taste peculiar to the grape-juice can be perceived; but it has acquired a spirituous flavour, together with a heating and intoxicating power, which was not in the