Allow it to dry; repeat the painting, and again allow it to dry. Paper so prepared is called "sensitive" paper. To show its properties, place upon it a piece of lace, a few fine feathers, or a sprig of fern, cover it with a sheet of window glass, and press the glass down with weights at the corners. The board may now be brought out of the dark room and exposed to sunlight, or to the strong diffused light of a bright day. The paper will soon begin to change colour, and after a time it will assume a rich brown tint. The whole arrangement must then be taken back to the dark room, the glass, ferns, &c., removed, and the paper immersed in clean rain water and well soaked, the water being changed a good many times. A beautiful image, in white on a dark ground, of the object employed will remain on the paper, which may now be dried and exposed freely to light, for the water will have washed away all the silver salt that has not been affected, and the picture will be permanent.

It has already been mentioned that the action of the sun's rays can also be produced by the rays from the electric light, and we have therefore an instance, to which many others might be added, of the production of chemical force from electricity. Even with the single cell described in Experiment 2, page 4, some striking illustrations may be obtained, as will be seen from the experiments which follow.

Experiment 5.--Attach a small strip of platinum foil to the end of each wire. Cut a card so that it will stand upright in a wine-glass, and divide it in two. Then fill the glass with the card in it with a solution of the salt called potassium iodide, and bring one of the platinum poles into each of the two divisions which the card makes. The pole which is connected with the copper plate immediately produces a brown colour on that side of the card. The electricity breaks up, or decomposes the salt into two substances: potassium, which owing to its action on water is not seen, and iodine, the substance which gives the brown colour in this experiment. Pour into the brown liquid on one side of the card a little of the smooth pasty liquid obtained by adding a good deal of boiling water to starch, stirring and cooling. The starch will combine with the iodine and form a beautiful blue colour.

Experiment 6.-Dissolve in some cold water as much of

the salt called copper sulphate (blue vitriol) as the water will take up, and substitute this solution for the potassium iodide used in the last experiment. The card is unnecessary. The salt will be decomposed, and copper will be deposited on the platinum pole which proceeds from the zinc plate. If the poles are reversed the copper will soon dissolve off the one platinum plate and appear on the other. This experiment is a simple example of the important manufacturing process called electroplating. Other metals, such for instance as gold and silver, may by analogous means be deposited from their solutions.

Experiment 7.-Decomposition of Water by Electricity.-With the assistance of a more powerful battery, and the little piece Fig. 1. of apparatus shown in Fig. 1, water may be decomposed into the two gases of which it consists. Thick platinum wires pass through the sides of a glass and terminate inside in flat plates (stout platinum wires hammered flat at the ends do very well). The holes in the glass can easily be made by a bradawl filed to a square point and constantly moistened with a mixture of turpentine and camphor, and the wires may be cemented in the holes with sealing-wax. The glass is filled with water to which a drop or two of sulphuric acid has been added, and over each of the plates a test tube filled with the same acidulated water is inverted. The two ends of the wires are now connected with the poles of the battery. In an instant bubbles of gas rise from the two plates into the test tubes. The gas which comes from the pole connected with the zinc is called hydrogen, that from the other oxygen; and it will be observed that the volume of the former is twice as great as of the latter. When a little gas has been collected, the mouths of the tubes may be closed with the thumb, the tubes removed, and the gas examined with a taper in the manner described under their respective names.

This is the most important instance of the production of chemical force at the expense of electricity that we possess. The electricity disappears during the experiment, but the hydrogen and oxygen which are produced are able, as we shall find hereafter, to exert an enormous force when they once more combine together.

Lastly, let us study the forces which are called into existence during the exertion and consequent disappearance of chemical force. We have, in point of fact, just been considering one of the most interesting cases of this kind in the voltaic battery. In the battery, the electricity which is called into being proceeds directly from, and is proportionate in amount to, the chemical force which is destroyed in the cell. The zinc is constantly being dissolved by the sulphuric acid, and the force which is thereby exerted immediately takes the new form of electricity. From this electricity we can, as we have seen, produce heat, light, and finally, chemical force, so that we can ultimately recover the very same form of force that we lost in the battery. One of Faraday's greatest discoveries proved that there is no loss of force throughout these changes, but that, provided we can prevent the force from assuming other forms, the chemical energy produced by the decomposition of the water is exactly equal to the energy lost in the battery.

One or two simple experiments will illustrate the direct production of other forces from the chemical, but it is unnecessary to multiply them, because the whole study of chemistry is full of such illustrations.

Experiment 8.-Take a piece of phosphorus, about the size of a small pea, and about an equal quantity of iodine. Place them side by side on a slate or a piece of tin-plate, and push the phosphorus with a knife till it touches the iodine. Chemical action is instantly exerted, and the mass bursts into flame, producing thereby light and heat.

Experiment 9.-Warm two tumblers: put into one a few drops of strong ammonia (spirits of hartshorn), and into the other a few drops of hydrochloric acid (spirits of salt). Each will give out a gas, and when the mouths of the tumblers are held together these gases will combine, and a dense white smoke will be produced, which will before long settle down on the inside of the glasses in the form of a white solid powder (sal-ammoniac). The chemical action is in this case accompanied by the exertion of the force of cohesion. The lost force takes the form of heat, which, how ever, can only be perceived by the thermometer.

When gunpowder or any other explosive substance is burnt, it is converted into gas; very energetic chemical

action takes place, and a great deal of force is consumed. But the force is entirely reproduced, for the cohesion of the solid is destroyed, a great expansion, which is nothing but a motion of the particles, takes place, and heat and light are evolved. Every one knows what powerful work the expansion may be made to do. Heavy shot can be thrown from a gun to an enormous distance, and solid rocks torn in fragments by the burning even of a small weight of the powder.

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The result of the researches of the last quarter of a century has been to convince most scientific men that, various as are its manifestations, there is in nature but one force, namely, MOTION motion of masses, motion of particles, motion in lines, motions of rotation and vibration, and motion in other modes. As regards heat and light this strange doctrine may be said to be already demonstrated, for these remarkable forces have been proved to be merely peculiar modes of motion in the particles of matter. And the close connection

of heat with the other forms of force forbids us to doubt that its nature is essentially similar to theirs, and that they also are modes of motion. Science has still a great work before her in the investigation of force, but enough has already been accomplished to enable us to take a far wider view of the kingdom of nature than was possible to our fathers, and to reveal to us something of the unity and simplicity that underlies the marvellous complexity of the universe.

OBJECTS OF CHEMICAL INQUIRY.

It has already been mentioned that the science of chemistry is concerned with the operations of chemical force. There are four chief points to which the attention of the chemist is mainly directed in his study of the solid, liquid, and gaseous substances which are met with in nature.

It

1. Their composition.-Take a piece of bone. How is it affected when strongly heated in a furnace ? It becomes whiter, lighter, and less solid than before (bone-ashes). But how is it affected when heated in a covered vessel? becomes lighter, and black (bone-black). boiling water, or to steam, how is it affected? It becomes lighter, and remains white; but in the water is dissolved gelatine. How in hydrochloric acid? It becomes trans

If exposed to

parent; the bone-earth is dissolved, and a gristly mass remains, which, when boiled with water, turns to gelatine. What is the action of fire upon the gelatine? In a covered vessel it is converted into charcoal, in an open one it burns and disappears. These few experiments show that the bone contains gelatine which is combustible, and an earth which is not so; they show at the same time that it is the carbonized gelatine which, in the second experiment, colours the boneearth black, and makes it bone-black; that this gelatine dissolves in water, but not in hydrochloric acid, &c. Gelatine and bone-earth are called the proximate constituents of bone, but by continued chemical processes these can be resolved still further, that is, separated into simpler constituents. In bone-earth are found phosphorus, a metal (calcium), and oxygen; in the gelatine, besides carbon, three other bodiesoxygen, hydrogen, and nitrogen. These bodies can be decomposed no further by any known method of analysis, and are therefore called simple bodies, or elements. There are now about sixty known elements, and almost every year adds to their number; but this increase is of little importance to chemical science or its applications, for it consists of elements which but very seldom occur. This separating of compound bodies into simple ones is designated by the name of decomposition, and the process of ascertaining the composition of any substance is called analysis.

When a substance contains two or more elements, held together by chemical force, it is called a compound. Compounds are always quite different in properties from their constituents.

When a substance contains two or more other substances (elements or compounds), not held together by chemical force, but present as it were accidentally together, it is called a mixture. In a mixture the properties of the separate ingredients are still perceptible.

This distinction between element, compound, and mixture is very important. The following illustrations will assist to fix it in the memory. Water is a compound of the elements hydrogen and oxygen, which are held together by chemical force. Air is a mixture of the elements oxygen and nitrogen, which are not in chemical combination with one another. Gunpowder is a mixture, containing the elements carbon and