THE FIELD OF CHEMISTRY.

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The mass

heated, a chemical reaction is started. becomes very hot, and glows brightly. When the product of the reaction is examined, it is found to be neither iron nor sulphur, as judged by its properties. The magnet does not attract it. Carbon disulphide does not dissolve the sulphur. The iron and sulphur have united to form a compound, ferrous sulphide (cf. § 106). They might have been ground together in any proportion to form a mixture,.but to form ferrous sulphide they unite in the proportion of 7 parts, by weight, of iron to 4 parts of sulphur. Any excess of either would simply be left over, uncombined.

We may summarize the differences between mixtures and compounds thus:

1. In a mixture the ingredients may be present in any proportion. In a compound the elements are united in definite proportions (cf. § 65).

2. When a mixture of elements is made there is generally no energy change. The formation of a compound is usually accompanied by an energy change, i. e., evolution or absorption of heat (cf. § 27).

3. The ingredients of a mixture may be separated by mechanical means. The elements of a compound can be separated only by some chemical change.

Mixtures are sometimes called physical, or mechanical, mixtures.

II. The Field of Chemistry. - Both Physics and Chemistry deal with matter and its changes. But

Physics considers especially the changes in which the composition of the substance is not altered, while Chemistry is concerned with those that result in new substances. The study of substances and their properties is thus the particular field of Chemistry. Many substances occur in the earth, but many others must be made in the laboratory, or on a commercial scale. But the occurrence, preparation, and properties of elements and compounds are only a part of what we study in Chemistry. There are rules according to which substances react; these are the Laws of Chemistry (cf. §§ 65 and 94). To explain a rule or law we devise a theory, or hypothesis (cf. §§ 95 and 141).

12. Importance of Chemistry. - A knowledge of Chemical Science is necessary for the intelligent study of other natural sciences, such as geology, astronomy, biology, physiology, etc., for these sciences make special applications of the ideas of Chemistry. Chemistry is also very practical, for the facts and methods it teaches are in the most common use in the arts and in every-day life. It finds application in medicine, in sanitation, in domestic science, in the extraction of metals from their ores, in the refining of petroleum and coal tar, and in the manufacture of steel, illuminating and fuel gas, paints, dyestuffs, food products, ice, alcohol, soap, glass, paper, explosives, etc.

EXERCISES.

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13. Exercises.

1. Classify the following as either physical or chemical changes: the souring of milk, the burning of wood, the evaporating of water, the tarnishing of silver, the dissolving of sugar in water, the bleaching of muslin, the melting of lead.

2. Give all the physical properties you know of the following substances: lead, wood, sugar, coal, gold, iron.

3. Give the chemical properties of the substances of 2.

4. What reason is there for considering silver an element? 5. Name some physical changes brought about by means of electricity, of light, and of heat. Name some chemical changes brought about by the same agents.

6. Learn the metric tables for length, volume, and weight (Appendix i).

7. How many cubic centimeters in a liter? State the relation between g., mg., dg., cg., kg. cm., dm., and m.

In 1 c.dm.? Between mm.,

8. In what way are the metric units of length, weight, and volume connected?

9. What is the weight of 106 c.c. of water? Give the volume, in c.c., of 30 g. water. Of 30 g. platinum of S.G. 21.5. Of 30 g. ether of S.G. 0.72.

R

CHAPTER II.

OXYGEN.

14. The Air and Chemical Change. Since air is a gas, and hence difficult to handle, men were slow in learning how it is connected with the important chemical changes that take place only when it is present. Among these changes are the respiration of animals and plants, the fermenting of fruit juices, the decay of wood and other organic materials (cf. § 25), the tarnishing of metals, and the burning of combustible substances. When the relation of air to these changes was once recognized, the progress of science was rapid.

15. Rusting of Metals. Most metals undergo tarnishing, or rusting, either at ordinary or at elevated temperatures. This had been known for many centuries, but it was Jean Rey who first noticed, in 1630, that given amounts of the metals increase in weight when the metals rust, and who first stated that contact of the metal with air was the probable cause. Other investigations led to the same conclusion. However, Rey's explanation of rusting was not accepted and applied until restated

PRIESTLEY AND LAVOISIER.

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by Lavoisier, in 1774. Lavoisier sealed a weighed quantity of tin in a flask, and heated it for several days. He noticed that the tin was partly changed into the white powder which had been studied by Rey. Although a chemical change had evidently occurred inside the flask, no change in weight could be noticed. But when the flask was opened, a certain amount of air rushed in. This showed that the tin rusted by taking up something from the air in the flask.

16. Priestley and Lavoisier. In the same year in which Lavoisier made his important experiment with tin, Priestley, in England, prepared the gas which tin and other

metals take up from the air. Priestley filled a glass vessel (Fig. 2) with mercury and floated various substances to the top of the mercury. He then heated them by means of a burning glass. One of the substances was the red powder, mercuric oxide.

It can be made

by heating mercury

FIG. 2.