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5. The Measures Of Weight are based on the Gramme, which is equal to 15-432 grains. The table is as follows:—

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The temperature of substances to be weighed or measured is of great importance in Chemistry, and is generally reckoned by the Centigrade

thermometer. The following is a comparative table of the scale of the Centigrade and that of Fahrenheit's thermometer.

C. F. C. F. C. F. C. F.

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II.—Chemical Combination.

1. We all know what it is to mix two or more substances together, and, indeed, come in contact with such mixtures at every turn, in the details of common life. But chemical combination is very different from mere mixture. The salt and pepper on our dinner-tables, for instance, sometimes get mixed, but each of the two still retains its own properties, so that the taste of the mixture is partly that of the one, partly that of the other. We may, moreover, mix them in any proportions we please, as, indeed, the cook does in seasoning her dishes. The particles of each, moreover, remain just as they were, the salt is still salt, and the pepper, pepper. And though perhaps in this case it would be difficult to separate the substances again, yet in the case of most mixtures it is possible to do so, and to bring them back to their original distinctions, just as they were before. For instance, when yellow ochre is mixed with water, to make the walls of an outhouse clean and tidy, the ochre needs to be stirred up continually to keep it from separating of its own accord, and if the pail in which the mixture has been made be left alone for a time, the water will all pass off in vapour by the heat of the sun and the ochre will remain at the bottom. Mixtures are only mechanical combinations, then, not real union of the substances composing them.

2. Chemical Combinations are quite different. When they take place the substances which are brought together lose their form and make other substances, different in appearance and properties. They are no longer distinct, but become lost one in the other, so that they need new names, and would not be suspected to be composed as they are.

Thus, for example, chemists can take two gases, oxygen and hydrogen, and change them into water, by exploding them when mixed. If you slake a piece of lime in a pail of water and having let the water settle till it be perfectly clear, then draw it off, you might fancy that it was just the same as it had been. But, instead of being Bo, you will find, if you cause some carbonic acid gas to pass through it, that it has entered into chemical combination with a part of the lime, for the gas will make it cloudy by separating this lime from it, and the bottom of the glass in which it stands will soon be covered with white chalk.

3. This property of chemical combination, or chemical affinity, as it is often called, is not universal, for some substances, as for instance fluorino and oxygen, have never been made to combine with each other, and have never been found in combination.

Some substances, moreover, have a great affinity for some others, and less affinity, or none, for all besides, as if they had their likings and aversions, as we ourselves have. Thus, chlorine has a great affinity for hydrogen, and some of the metals, such as calcium, potassium, and sodium have so great an affinity for oxygen, that, if you let them touch water, they will at once take all the oxygen out of it, and unite with it so violently as to catch fire.

4. No merely mechanical means will separate bodies chemically combined. Marble, for instance, consists of three bodies, carbon, oxygen, and calcium ; but you may grind a piece of it into the finest dust, and yet each particle of the dust will be as truly marble as the whole piece was. It needs chemical power to separate the elements of which the whole consists, and this is found in the effects of heat. When made brightly red-hot, the carbon, and part of the oxygen, are driven off as a compound gas, and the calcium, and the rest of the oxygen, remain behind, in the form of lime. By taking still further trouble, the chemist can make the separation of all three elements complete, and by taking away the oxygen, leave only the metal calcium. But you see that even by heat alone, two new compound bodies were formed out of the original one—the marble.

In the same way, as you already know, oxygen and hydrogen remain unchanged and only mixed, till a spark, by exploding them, brings into play a new chemical force which changes them into water.

5. It is the same with gunpowder, which is a mixture of nitre, charcoal, and sulphur, each, first, ground very fine, and the three then blended thoroughly together. Yet, however finely ground and thoroughly mixed, the particles of each substance remain unchanged, and can even be seen, if a grain of the powder be examined under a microscope. There is no chemical combination. But, if a spark fall on the mixture you know what follows: there is a flash, and a cloud of smoke, and the powder is gone. It has changed into new substances, several of which are gases, and none are in any way like the powder itself.

Among the effects of chemical combination a very striking one is seen in the change of colour it often causes. Bright red vermilion colour is produced by heating yellow sulphur and white quicksilver together, and the union of black charcoal with sulphur makes a compound that has no colour at all.

6. The very same elements, moreover, often form, in different combinations, the most dissimilar substances. Thus gum, arrowroot, and sugar, are, all alike, composed of charcoal, hydrogen, and oxygen.

It is the same with the smells of bodies. Many of our finest perfumes such as oil of bergamot, oil of citron, and otto of roses, are compounds of charcoal and hydrogen, which, themselves, are scentless.

It is to he specially noted that however great the changes effected by new chemical combinations, they cause no loss of weight. If, for example, the carbon, &c., given off in the burning of a candle be carefully weighed, it will be found that the whole not only weigh no less, but are heavier, than the candle itself. The oxygen which has been consumed in the burning is the cause of this.

7. Unlike mechanical mixtures, in which the proportions of the compound bodies may be varied at pleasure, those of The Substances Entering

INTO CHEMICAL COMBINATIONS DO SO ONLY IN FIXED PROPORTIONS. In these

and in no others can they be made to unite. This is one of the fundamental laws of chemistry, and must be carefully kept in mind.

Thus, water always contains two parts of hydrogen to one part of oxygen: ammonia always contains three parts of hydrogen to one of nitrogen; common salt always contains 35J parts of chlorine to 23 of sodium; and marble, 40 parts of calcium to 12 of carbon, and 48 of oxygen. Wherever you find them,these substances are always the same; —in these ratios and in no others will their components combine to form them. Water in Australia is the same as in England; common salt is the same in India and Patagonia; ammonia yields the same analysis wherever tested; and a piece of marble from Italy is the same as a piece of marble from the quarries of Nova Scotia.

8. A second law of chemical combination is that When Elements Unite

IN DIFFERENT PROPORTIONS, THE HIGHER PROPORTIONS ARE ALWAYS MULTIPLES OF THE FIRST OR LOWEST.

In the five oxides of nitrogen we have an excellent illustration of this law. The proportion of nitrogen is the same in them all.

Nitrous Oxide contains in every 44 parts, 28 of nitrogen and 16 of

oxygen, or 2 atoms of nitrogen and 1 of oxygen. Nitric Oxide, in every 60 parts contains 28 of nitrogen and 32 of

oxygen, or 2 of nitrogen and 2 of oxygen. Nitrous Anhydride contains in every 76 parts, 28 of nitrogen and

48 of oxygen, or 2 atoms of nitrogen and 3 of oxygen. Nitrogen Peroxide, in every 92 parts contains 28 of nitrogen and

64 of oxygen, or 2 atoms of nitrogen and 4 of oxygen. Nitric Anhydride, in every 108 parts, contains 28 of nitrogen and

80 of oxygen, of 2 atoms of nitrogen and 5 of oxygen.

The proportions of oxygen in all these cases will be found to be the multiples of 16—by 2 = 32; by 3 = 48; by 4 = 64; and 5 by = 80.

The law is universal. If a substance combine with more than 16 parts of oxygen, the quantity must be 32 parts, and if it go beyond 32 the next quantity must be 48, and, if it go beyond that, the next must be 64.

These are the two laws of most importance for beginners in Chemistry.

III.—Atoms, Atomic Weight, And Molecules.

1. In all books on Chemistry, the words Atom, Atomic Weight, and Molecule constantly occur, and must be explained if the student is to understand what he reads.

Chemists assume that all elementary bodies are composed of very small and invisible particles which are called Atoms, from the Greek word atomos that which cannot be divided, and these atoms are assumed to be of different weights in different elements. This is, of course, so far, only a theory, for no one ever saw an atom or separated a substance into atoms. Yet it is a theory which accords so well with facts that it has been received by all scientific men since the days of its first publication by Dr. Dalton.

2. The difference in weight of the different kinds of atoms is supposed to account for the different powers of combination they show, all compounds being only the union of atom with atom. All atoms of any one element are supposed, however, to be always exactly alike both in size and weight.

3. It is by the aid of this theory that chemists determine the relative weights of elementary bodies and their compounds. Hydrogen, which combines in the proportion of two parts, or the multiples of two, with other bodies, is taken as the unit, and the weight of other substances is estimated by the difference between their power of combination and those of this gas.

4. Calling hydrogen 1, the weight of oxygen is 16, because the combining proportion of oxygen is 16 times greater than that of hydrogen. Hence the weight of an atom of oxygen is assumed to be 16 times that of an atom of hydrogen. Twelve parts of carbon are the smallest proportion that can exist in a compound, and therefore an atom of carbon is assumed to be 12 times heavier than one of hydrogen. Chlorine has for its lowest combining proportion 35-5, and therefore this is assumed to be the weight of an atom of it. Iron has a combining proportion of 56, and therefore this is taken as the weight of an atom of iron. The atom of sulphur, in the same way, is 32 times heavier than that of hydrogen, and that of nitrogen 14.

i 5. The term Molecule is given to the smallest quantity of an elementary substance which is supposed to be capable of existing in a separate form, just as an atom is the name given to the smallest indivisible particle of each element which can exist in a compound, united with other particles either of the same element or of some other. Molecule has, therefore, to do with volume, or bulk, just as atom had to do with weight.

6. The volume of the molecule of a compound body in the form of gas is exactly double the volume of the atom of hydrogen: thus

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