This coating becomes itself coated by being the terminal plate of the negative pole. The model itself serves to give this coating the required shape, but is not otherwise of any value in the operation. Care must be taken to continue the plumbago coating to the wire of the battery, so as to form a continuous conducting line. SUMMARY. Sound, heat, light, &c., are the accompaniments of force, not the terminations of it. Page 357. We can neither create nor destroy force, the origin of which is unknown to us. Page 357. We can apply force only by means of media. Page 358. The steam-engine is an example of the use of media to apply force. Page 358. Combustion is an example of force acting in a manner not yet accounted for. Page 359. Coal is the most convenient fuel. Page 359. Water is the most convenient medium of force. Page 359. Electro-plating is also an example of the utilisation of force. Page 360. CHEMISTRY IN ITS RELATION TO PHYSICS. (1.) Introduction.-In speaking of the causes, effects, and nature of the phases of energy called Heat, Light, Electricity, &c., we have had frequently to talk of chemical composition and decomposition. Probably most of my readers are already_more_or less familiar with Chemistry, but some few may not be. For these I have written a few pages, which I trust will be excused by those who do not need them. (2.) Composition of Water.-Take a saucer nearly full of water, and throw on it, lightly, a small piece of pure potassium. Immediately it will burst into flame, burn with a violet-coloured light, and be consumed. Contact with the water is the cause of its being set alight, and in burning it is absorbed by the water, which afterwards tastes differently from pure water, having somewhat the flavour of soda. But it is not that the water has had any effect on the potassium, but the reverse. The potassium has really affected the water, and broken some of it up into its constituent elements. Water is made up of Oxygen and Hydrogen, and Potassium has so much attraction for Oxygen that it will draw it out of the water, and leave only Hydrogen. That is, each drop of water, as it touches the Potassium, ceases to be water, and is broken up into Oxygen and Hydrogen. The Oxygen unites with the Potassium, while the Hydrogen passes away as gas into the air. The potassium having taken the oxygen into union with itself is no longer pure potassium, but potassic oxide, and this being dissolved in the water gives it the flavour spoken of. Pure Sodium will also, in a similar manner, separate, by mere contact, Water into its constituents, Oxygen and Hydrogen. The sodium will become sodic oxide by union with the oxygen obtained from the water, while, as before, the hydrogen will pass away as gas, but the water must cease to be water before the sodium can be combined with the obtained by its decomposition. oxygen We prove the presence of hydrogen in water in this way, by means of potassium or sodium, since we can easily collect the gas given off while the oxygen of the water is uniting with the potassium or sodium, and it proves to be pure hydrogen, which cannot have any source but the water. In the same way, by uniting water and pure chlorine at a high temperature, we enable the chlorine to combine with the hydrogen of the water, and the oxygen is set free, and can be collected as gas. Thus we can get from water both pure hydrogen and pure oxygen. We know, therefore, that water contains both hydrogen and oxygen, and we can also prove that it contains no other element. For we can convert any given quantity of water into these two gases, and find nothing remaining. If we use potassium, we only get the hydrogen free, the oxygen being combined with the metal; if we use chlorine, we get only oxygen free, the hydrogen being combined. But if we subject water to the action of a galvanic current, we get it entirely converted into gases, each of which is quite free and pure. The means by which this is effected are described on page 200; it will suffice to say here that the two wires c c conduct a galvanic current through the water at m, and the result is its decomposition and the rise of the gases oxygen and hydrogen in the tube ɑ. Fig. 207 is the apparatus seen from above; fig. 208 the same seen sideways. The arrows show the direction of the current, and b is a scale to show the quantity of gas evolved. That water is composed of hydrogen and oxygen, and only of these, may be still further proved by combining them again, and reconverting them into water, which may be done by passing through the mixed gases a galvanic current. So that water can be separated into its elements, and these may be reconverted into water by the same means -i.e., electricity. But we cannot take either hydrogen or oxygen from water without taking both. We must consider water as made up in exceedingly small pieces, much smaller than the smallest drop we could take up on the point of a needle. Each of these minute drops is made of hydrogen and oxygen, and when we get either gas it is by breaking up some of these drops into their elements. If we take a lump of ice and pound it into the finest powder possible, each of these grains can be melted into water, from which hydrogen and oxygen can be obtained. Fig. 209 shows, on a greatly magnified scale, the oxygen and hydrogen constituents of water, the two uniting to make the apparently single substance, water. OH OH OH OH OH OH OH OH The two gases are also always united together in exactly the same proportion. If we decompose any given quantity of water into its elements, we may measure these elements either by weight or by volume. If by weight, we shall find the oxygen eight times as heavy as the hydrogen. If by volume, we shall find the hydrogen twice as much as the oxygen. Thus the oxygen is much heavier than the hydrogen in fact, any volume of oxygen weighs sixteen times as much as the same volume of hydrogen. If, now, we put in the same vessel one ounce of oxygen and one-eighth of an ounce of hydrogen, we should find the hydrogen (though so much the lighter) occupied twice the room that the oxygen required. So, also, if we put the gases together so that the hydrogen occupies twice the room of the oxygen, the oxygen would be found to weigh eight times as much as the hydrogen. In either case the two gases could be converted into pure water, which would weigh exactly as much as the two gases together, but would occupy very much less room. O HO HO HO HO HO HO HO H : If an ounce of hydrogen and an ounce of oxygen be put together, the result will not be two ounces of water. The ounce of oxygen would unite with one-eighth of the hydrogen to make water weighing one ounce and one-eighth, while the remaining seven-eighths of an ounce of hydrogen would be left free and unaffected. So if we put together equal volumes of the two, the hydrogen will unite with half the oxygen, leaving the other half free. The hydrogen and oxygen of which water is made, differ very much in their natures from water. Hydrogen will burn, and oxygen enables combustibles to burn more vividly than they can in ordinary air. Water neither burns nor assists combustion; on the contrary, it is usually looked on as the natural enemy to fire. Oxygen and hydrogen are both gases, and therefore both invisible; water is a visible fluid. Water is heavy and falls to the ground; its constituent gases are light, and rise in the air. (3.) Chemical Elements.-Oxygen and Hydrogen are very important in a chemical view. They enter into combination with very many other elements, forming compounds of great chemical importance. Thus oxygen combines with very many of the other elements, just as we see it does with potassium and sodium, These combinations are called oxides, as— Oxide of Potassium or Potassic Oxide. Oxide of Sodium or Sodic Oxide. Oxide of Iron or Ferric Oxide. Oxide of Zinc or Zincic Oxide. Oxide of Lead or Plumbic Oxide. Hydrogen also combines with many of the other elements, These gases, oxygen, hydrogen, chlorine, nitrogen, and the solid, carbon, differ from water or any other compound substance, in that they are entirely simple. Out of water we can get oxygen and hydrogen; out of ammonia we can get hydrogen and nitrogen. But from pure carbon we can get nothing but carbon, out of hydrogen nothing but hydrogen, and so on of the other elements. Each is homogeneous, and incapable of further analysis by any power at present in our hands. Fig. 210 shows the simplest method of decomposing chemical Fig. 210. compounds. The method is described at page 185. The number of these ultimate constituents (of various combinations, of which the whole world is made up) is not large. Some sixty simple substances comprise the elementary materials of the universe. Of these, one-third may be considered of primary importance, being very widely spread, and necessary for the continuance of the earth, water, and air, and for the preservation of life. Another third may be classed as of secondary value, being of much less extent, but of great economic value, such as zinc, tin, gold, silver, &c. The remaining third are of but little general importance. The following table gives the names of the elements of first and second rate importance, the more important being printed in larger type than the other : |