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Gases combine in very simple proportions. Thus this combination by volume takes place-as in the case of chlorine and hydrogen in the proportion of 1 to 1, as in oxygen and hydrogen in that of 1 to 2, or as in the case of nitrogen in that of 1 to 3.

7. The atomic volume of gases and vapours is calculated from their respective specific gravity as compared with that of hydrogen or atmospheric air. The atomic weight of a gas or vapour is the weight of a volume of any one in particular, equal to the volume of any quantity of hydrogen the weight of which is taken as unity. The atomic volume of a body is the space occupied by a quantity of it proportional to its atomic weight.

All this may not be very clear, but it is not possible to make it simpler without going into too lengthy explanations. The whole subject must be studied after a considerable progress in chemistry has been made, to be easily intelligible.

8. SPECIFIC GRAVITY is the name given to the weight of an equal volume of one substance in comparison with that of some other.

If we weigh one gramme of sponge, and compare its bulk with that of the weight itself, in the opposite scale, the sponge will, of course, be very much the larger. In the same way a piece of sponge just the size of the weight will be very much lighter than it.

9. It would evidently be inconvenient, if always possible, to ascertain the comparative size of equal weights of a substance, and it is much more simple to test the comparative weight of equal sizes of it. To do this, some one substance must be chosen as the standard by which to ascertain the comparative weight of equal bulks of other substances.

10. Water is universally accepted as this standard for all liquids and solids, and atmospheric air or hydrogen is taken as that for gases and vapours. The weight of any body compared with these standards is called its " specific gravity," or its density, which is the preferable expression.

11. The density or specific gravity of a gas is ascertained by a very simple means. A large glass globe, furnished with a stopcock, is weighed when full of dry air. It is then thoroughly exhausted by an air-pump and weighed again. Its weight now, when empty, is next subtracted from its weight when full of air, and thus the weight of the air it contained is ascertained.

It is now filled with the gas to be weighed, and when the weight of the empty glass is taken from the result, we have the clear weight of the gas. The volume of air and of gas must, of course, have been the same, and thus we know how much the one was more or less than the other. It is necessary, however, to see that the pressure of the air, as shown by the barometer, and the heat as shown by the thermometer, be the same throughout.

12. To ascertain the specific gravity of liquids all that is necessary is to

take a dish of any convenient size, the weight of which we know, and find out how much heavier or lighter it is, first, when filled with water, and second when filled with the liquid to be weighed, the liquid being of the same temperature as the water.

13. The specific gravity of solids is ascertained by taking advantage of the law of liquids, "that a solid immersed in a liquid loses a weight equal to the weight of an equal volume of the liquid." It is, hence, only necessary to weigh a piece of the solid in the air, and then put it in water and weigh it again there.

CHEMICAL SYMBOLS AND FORMULE.

1. Instead of writing out the names of all substances in their science, in full, chemists have agreed upon a system of chemical notation by which much labour is saved. The first letter of the Latin name of each substance is taken as its chemical symbol, except where there are two or more elements beginning with the same letter, in which case the second letter of each name is added to the first. Thus F stands for Fluorine; Fe for Iron, from Ferrum, the Latin word for it; S for Sulphur; K for Potassium from Kalium, its Latin name; Na for Sodium, from Natron, its Latin name; and Pb, not L, for Lead, from its Latin name, Plumbum.

Remember that when the first letter of two names is the same, a distinction is made by means of the second: thus the symbol of Carbon is C, of Chlorine, Cl; of Copper, Cu (from Cuprum), and so on.

2. These letters or symbols represent the weight of the smallest quantity of the element for which they stand, which can enter into combination with another element, and thus they serve a double purpose. For example, the symbol H always stands for 1 part, by weight, of hydrogen, O for 16 parts, by weight, of oxygen, C for 12 parts of carbon, S for 32 parts of sulphur, and so on; these being the smallest number of parts of each of these elements which can exist in a compound. They represent the atomic weights of the elements, each element being supposed, as we have seen, to consist of atoms of exactly the same size and weight in the same body. If hydrogen be taken as 1 these figures thus show the weight of the substances to which they are affixed, compared to that of the atom of hydrogen.

3. Compound substances are represented by placing the symbols of the elements of which they consist, together. Thus C O stands for the molecule of carbonic protoxide, a compound substance consisting of single atoms of carbon and oxygen: Pb and O for a compound of single atoms of lead and oxygen. This combination of symbols is called a FORMULA.

When more than two substances unite to form a compound, the symbols of each are given, but it is not necessary to illustrate this here.

The following is a Table of the

ELEMENTS, WITH THEIR SYMBOLS AND ATOMIC WEIGHT. Each line should be read thus: "The aluminium atom is represented by the symbol Al, and weighs 27.5 times as much as the hydrogen atom."

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Those in italics are rare and are not used to any extent in arts or manufactures.

SPECIFIC HEAT.

i. When we heat equal weights of different substances, we find that the amount of heat required to produce an equal temperature varies in the case of each. Some have a greater specific or special capacity for heat. It is this peculiarity which is meant by the shorter expression specific heat. The greater the capacity for heat, that is, the more heat it takes to pro

duce a given result, the greater the specific heat-the less heat required, the less the specific heat, of any body.

2. Illustrations of the different specific heat of different substances may be found on every hand. Thus if you take equal quantities of water, oil of turpentine, iron, and mercury, at the same temperature, and apply an equal amount of heat to each, you will find that the turpentine will grow twice as hot, the iron eight times as hot, and the mercury thirty-three times as hot in the same time, as the water does.

3. If, again, you try to melt ice with equal quantities of different bodies which have all been heated to the same point, you will find that much less will be melted by some of them than by others. A pound of any of the substances just named, will have much less effect than a pound of hot water--though the heat of each when brought in contact with the ice has been the same. The reason, of course, is, that, as they required much less heat than water does to raise them to any given temperature, they have just so much the less to give away. For the same reason some bodies cool much more quickly than others of the same temperature. If two vessels, one with mercury, the other with water, be left to cool, the mercury will fall in temperature twice as fast as the water.

4. If we take two given weights of water at different temperatures and pour them together, the heat of the whole will be just the mean, or middle, heat between that of each. Thus a pint of water at 200°, mixed with a pint at 100°, would give us two pints at 150°. The colder water would gain and the warmer would lose 50° of heat. It is the same when we mix two portions of the same liquid, whatever it may be.

5. But if we mingle a pound of mercury at 160° with a pound of water at 40°, the temperature of the compound will not be the mean or middle of the combined heat, as it would have been had mercury been mingled with mercury, or water with water. Instead of 160° + 40° = 2002 100° for the whole, it will be only 45°, the mercury having lost 115° and yet having heated the water only 5°. If, on the other hand, we mingle a pound of water at 160° with a pound of mercury at 40°, the whole will show a heat of 155°, the 5° lost by the water having heated the mercury 115°.

Thus the same amount of heat exhibits a higher intensity when contained in quicksilver than when contained in water.

The specific heat of solids is most easily ascertained by heating them to a given point, and then plunging them into water or any liquid at a lower temperature.

6. The standard chosen for the comparison of the specific heat of different bodies is water, which has a greater capacity for heat than any other body. Weight rather than measure is used in the comparison, and water being fixed as equivalent in specific heat to 1000, the specific heat of other substances is expressed by some lower figures, according to the facts in each case.

The following is a table of the specific heat of various substances :

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8. The great amount of heat required to raise the temperature of water has the effect of keeping the sea at a comparatively equal degree of heat all round the year. It cannot be suddenly raised in temperature to any great extent, and, thus, life, both animal and vegetable, is not exposed to the fatal results which must follow if it were otherwise. If it rose in warmth as easily as mercury, the dry land would be speedily turned into a lifeless desert.

From the same cause, the sea is of the greatest use in tempering the cold of the land. It cools so slowly, and gives off so much heat as it does so, that the air is warmed by it, and abates the rigour of winter in the regions over which it passes. It is from this reason that an island is always so much more temperate than a continental country, in the cold latitudes. Britain has a much milder climate than the parts of Europe equally far north, and Vancouver's Island is very much milder than the land on the Continent near it.

9. The specific heat of equal weights of different substances, as we have seen, varies greatly; but if instead of taking equal weights we substitute quantities, proportioned to the respective atomic weight of bodies, the striking fact is brought to light that the amount of heat these bodies are capable of receiving, is identical, when their atomic weight is kept in view in the quantities employed. Thus the table already given, shows that the atomic weight of sulphur is thirty-two, and that of iron fifty-six. If, now, thirty-two parts of sulphur, and fifty-six parts of iron, be heated to the same degree, and then put into the same quantity of water, say at the freezing point, the thirty-two parts of sulphur and the fifty-six parts of iron, will each raise the heat of the water to the same extent, and it is the same with other substances treated in the same way. Thus, atom for atom, all bodies have the same capacity for heat, though, weight for weight, that capacity varies.

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