collected over water. Potassic chlorate is a compound of chlorine, potassium, and oxygen, and its symbol is KClOg. The oxygen is set free by the heat, leaving the chlorine and potassium as a compound called potassic chloride.

A chemical change of this kind is called a reaction, and may be expressed in the form of an equation. In this case the equation will be —

KCIO2= KC1+0。

indicating that the potassic chlorate, KClO3, has been broken up into potassic chloride, KCl, and oxygen, O.

If a small quantity of black oxide of manganese be mixed with the potassic chlorate, the gas is given off at a much lower temperature, and more gradually. The manganese, however, undergoes no change in the process.

II. Properties of Oxygen.—We have already seen (2) that a taper burns much more vividly in oxygen than in the air.

If a piece of charcoal be lighted and plunged into a jar of oxygen, it burns with a very brilliant white light. Pour now some clear lime-water into the jar, and shake it. The liquid becomes milky white, showing that the gas has undergone a change; for oxygen has no effect upon lime-water. Charcoal is a form of the element carbon, and the oxygen has combined with it. The compound produced is called carbonic anhydride (carbonic acid), and its composition is represented by the symbol, CO2.

If a bit of sulphur be ignited and put into a jar of oxygen, it burns with a vivid blue light and a suffocating odor. The sulphur and the oxygen have combined to form sulphurous anhydride (sulphurous acid), the symbol of which is SO2. 2°

Phosphorus burns in oxygen with dazzling brilliancy, producing dense, white fumes of phosphoric anhydride (phosphoric acid), P2OË.

Thus far, we have found that substances which burn in the air burn more rapidly and more brightly in oxygen. If, now, we fasten a bit of wood to a steel watch-spring, ignite the wood, and plunge the whole into a jar of oxygen, the watch-spring takes fire, and burns as readily as a splint of wood burns in the air. the oxygen combine, forming a mixture of two oxides of iron, FeO, ferrous oxide, and Fe,O,, ferric oxide.

In this case the iron and

The above experiments illustrate several properties of oxygen: (1) the passive state in which it exists under ordinary circumstances, it being necessary to heat it in order to make it combine rapidly with any substance; (2) the intense energy with which it enters into combination when once aroused; and (3) the wide range of its affinities. It combines with every known element, except fluorine.

12. Ozone. — If a series of electric sparks be sent through dry oxygen, the gas undergoes a change. It acquires more active properties and a peculiar odor from which it takes its name, ozone.* The odor perceived when an electrical machine is worked is owing to the presence of ozone.

If a paper moistened with a solution of potassic iodide (iodide of potassium), KI, and starch paste be held opposite a point on the conductor of an electrical machine, the paper becomes blue. The iodine is set free by the ozone, and unites with the starch, forming a blue compound, which colors the paper. Ordinary oxygen has no such power to separate iodine from its combination with potassium.

There has been much discussion concerning the nature of ozone. It appears, however, to be simply oxygen in

a modified and condensed form.

* Ozone is derived from a Greek word which means to emit a bad odor.

This partially active state of oxygen is intermediate between the active and passive states already mentioned.

13. Allotropic States. There are other elements which exist in states in which their properties are as unlike as those of oxygen and ozone. These different states of an element are called allotropic states.

14. Oxides." The simple compounds of the elements with oxygen are called oxides, and the specific names of the different oxides are formed by placing before the word oxide the name of the element, but changing the termination into ic or ous, to indicate different degrees of oxidation, and using the Latin name of the element in preference to the English, both for the sake of euphony. and in order to secure more general agreement among different languages. When the same element unites with oxygen in more than two proportions, the Latin prepositions or numeral adverbs, sub, per, bis, etc., are prefixed to the word oxide in order to indicate the additional degrees. Formerly these compounds were called the oxides. of the different elements, the degrees of oxidation being indicated solely by the prefixes." *

The ending ous indicates a lower degree of oxidation than the ending ic.

15. Acids. - Very many of the non-metallic oxides combine with water, forming compounds which turn blue litmus-paper red. Such oxides are called acid oxides, or anhydrides, and their compounds with water are called acids. Thus SO2+ H2OH,SO. SO, is sulphurous anhydride, often improperly called sulphurous acid, a name which properly belongs to H2SO,. So H2O+SO, (sulphuric anhydride) = H2SO (sulphuric acid).

2

Here, too, as in the case of oxides, the ending ous denotes a lower degree of oxidation than the ending ic.

*First Principles of Chemical Philosophy. By Josiah P. Cooke, Jr., 1868, p. 69.

When more than two acids are formed by the same elements, the prefixes hypo (less) and hyper or per (more) are used. Thus, hydrogen, chlorine, and oxygen form four acids: HCIO, hypochlorous acid; HCIO2, chlorous acid; HCIO, chloric acid; and HCIO hyperchloric acid, or perchloric acid.

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It will be noticed that these acids are named from the element with which the oxygen combines to form an acid oxide; sulphuric and sulphurous acid from sulphur, and so on. It will be seen also that in the symbols of the acids the hydrogen is placed first, and the oxygen last. 16. Bases. Many of the metallic oxides combine with water to form compounds whose properties are the opposite of those of the acids. These are called basic oxides, and their compounds with water are called bases. The specific name of a base is made an adjective in ic or ous formed from the Latin name of the metal, followed by the term hydrate. Thus, (H2O + K2O) = HKO. K2O is potassic oxide, and HKO is potassic hydrate. So H2O+BaO (baric oxide) =H,BaO, (baric hydrate).

The potassic and sodic hydrates (HKO and HNaO) and some others are called alkalies. They differ from the remaining hydrates in that they cannot be decomposed by heat.

17. Neutrals. Those oxides which, like water, are neither acid nor basic, are called neutral oxides, òr neutrals.

18. Salts. A metal may take the place of the hydrogen in an acid, and the compound thus formed is called a salt. Thus, the metal potassium may take the place of the hydrogen in H2SO, and H2SO4, forming the salts K2SO, and K,SO. A salt is named from the acid, the ending ic being changed to ate, and the ending ous to ite, with the name of the metal prefixed with the ending ic or ous. Thus, K,SO, is potassic sulphite, and K2SO,

potassic sulphate. FeSO, is ferrous sulphate, and Fe23SO ferric sulphate.

19. Binary and Ternary Compounds. - Compounds like the oxides, which contain two elements, are called binary compounds; and compounds like the acids mentioned above (15) which contain three elements, are called ternary compounds.

HYDROGEN.

20. Preparation of Hydrogen.-The most convenient way of preparing hydrogen is by the action of zinc on dilute sulphuric acid. The apparatus required is shown in Figure 7.

Fig. 7.

The reaction between the zinc and the sulphuric acid is shown in the following equation:

H2SO1+Zn=Zn SO1+H2.

4

The zinc changes places with the hydrogen, forming the salt known as zincic sulphate, or white vitriol, while the hydrogen is set free.

21. Properties of Hydrogen.—If soap-bubbles are blown with a mixture of two measures of hydrogen to one of oxygen, they will explode violently when touched. with a lighted taper; showing the intense energy with