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3. Ammonia. — Fill with chlorine a long glass tube,
closed at one end. Close the other end with a cork, through which passes a dropping tube. (Figure 4.) If, now, a little ammonia be dropped into the tube, flashes of light and dense white
within. Next allow water to run into the tube as long as it will, and test the gas which remains, by putting a piece of litmus
paper into it.
The paper is
not bleached, hence the gas is not chlorine. Plunge a lighted taper into it, and it goes out, and the gas does not take fire ; hence the gas is neither oxygen nor hydrogen. It is nitrogen, and it always, as in this case, ex
tinguishes a lighted taper. This nitrogen must have come from the ammonia, since there was no nitrogen in the water poured into the tube.
The white fumes which appeared in the tube, when the ammonia was dropped in, showed that muriatic acid was formed. Now we have learned that muriatic acid is made up of chlorine and hydrogen. The hydrogen must also have come from the ammonia.
Ammonia, then, contains nitrogen and hydrogen. It has been found by experiment that it contains nothing but
these two gases.
4. Compounds and Elements. — We have now seen that muriatic acid, water, and ammonia, are made up of
other substances, which we can separate from them. In other words, they are compound substances, and we have decomposed them. But no means has been found of decomposing oxygen, hydrogen, chlorine, and nitrogen into simpler substances. These, and other substances which cannot be decomposed, are called elements.
Most substances with which we are familiar are compounds. The metals, however, are all elements.
There are only about sixty-five elements in all, and many of these are very rare substances.
5. Names and Symbols of Elements. - Elements have been divided into two classes, metallic and non-metallic. The following list contains the names of the most important elements usually put in each class, together with the abbreviations of these names commonly used. These abbreviations are called symbols.
It will be noticed that in some cases the symbol is an abbreviation of the Latin name of the element.
6. Affinity. — The force which draws elements together, and locks them up in compounds, is called affinity, or chemical force.
One of the characteristics of this force is, that it changes the properties of the substances which it brings together. It not only unites different substances, but unites them so as to form a new substance. It combines them.
This characteristic is well illustrated in the case of water, a substance wholly unlike hydrogen, which is combustible, and oxygen, which aids combustion.
The second characteristic of affinity is, that it always acts between definite quantities of matter. This may be illustrated by the following experiment: Partially fill the closed arm of the U tube (Figure 5)
with a mixture of hydrogen and oxyFig. 5.
gen, using more than twice as much
If now we put into the tube a mixture containing less than twice as much hydrogen as oxygen, the gas which remains after the explosion is found to be
oxygen. In both cases, the oxygen combines with exactly twice its bulk of hydrogen, and this will be true in whatever proportions the gases may be mixed.
This experiment also illustrates the third characteristic of affinity, which is, that it is often dormant until it is roused to activity by heat, or some other force. The
oxygen and the hydrogen showed no disposition to combine until the spark was sent through them.
7. Molecules and Atoms. - It is supposed that all bodies are made up of small particles called molecules, and that these molecules are made up of smaller particles, which are called atoms (that is, not to be divided), since we are not able to subdivide them.
In an elementary substance, the atoms are supposed to be all alike in form and weight, but those of one element differ in weight from those of another. In a compound substance, the molecules are supposed to be alike, each being made
of a fixed number of atoms of each of the component elements; but the molecules of one compound differ from those of another in their atomic constitution. This theory of the composition of substances is called the atomic theory.
Thus the molecules of muriatic acid are supposed to be made up of one atom of hydrogen and one of chlorine ; the molecules of water to be made up of two atoms of hydrogen and one of oxygen; and the molecules of ammonia to be made
of three atoms of hydrogen and one of nitrogen.
8. Symbols of Compounds. The symbol of a compound is made up of the symbols of its elements. It also indicates the atomic constitution of the molecules of the compound. The symbol of each element represents one atom of that element. If there is more than one atom of the element in the molecule, this is indicated by a small figure placed at the right of the symbol. Thus the sym-' bol for muriatic acid is HCl; indicating that muriatic acid is a compound of hydrogen and chlorine, and that a molecule of muriatic acid is made up of one atom of hydrogen and one of chlorine. The symbol for water is H,O, since water is a compound, whose molecule contains two atoms of hydrogen and one of oxygen. The
symbol of ammonia is H2N, its molecule being made up of three atoms of hydrogen and one of nitrogen.
9. Atomic Weights. - According to the atomic theory, an atom of chlorine weighs 35.5 times as much as an atom of hydrogen ; an atom of oxygen weighs 16 times as much as one of hydrogen; and an atom of nitrogen 14 times as much as one of hydrogen. If then we represent the weight of an atom of hydrogen by 1, the weight of an atom of chlorine will be 35.5; that of an atom of oxygen, 16; and that of an atom of nitrogen, 14. These numbers are called the atomic weights of these elements. (5.)
We now see that the symbol of a compound represents not merely the atomic constitution of the compound, but also the relative weights of its elements. Thus muriatic acid, HCl, contains i part by weight of hydrogen to 35.5 parts by weight of chlorine; water, H,O, contains 2 parts by weight of hydrogen to 16 of oxygen; and ammonia, HN, contains 3 parts by weight of hydrogen to 14 of nitrogen.
OXYGEN. 10. Preparation of Oxygen.- Oxygen is most readily obtained from potassic chlorate (chlorate of potash), a substance of whose weight it forms about 40 per cent.
The chlorate is heated in a flask (Figure 6), and the gas is