burst into a flame. Likewise, if a current of electricity be passed through water acidulated to render it a conductor, it will decompose the liquid into the two gases of which it is composed.

14. Metathesis, or Double Decomposition.-By far the greater number of chemical changes are not as simple as the two kinds already mentioned. More frequently two, and sometimes more than two, substances unite or react with each other, in which case both substances are decomposed and two or more new ones are produced, by a rearrangement of the elements contained in the compounds. Such a change is spoken of as "Metathesis" or "Double Decomposition." It may be illustrated by adding a few cubic centimeters of a solution of potassium iodide to one of mercuric chloride in a test tube. Two new substances are formed, both entirely different from the original, one of them now a brilliant red color and not soluble in water. Numerous illustrations of this kind of change will be had from time to time. Really, metathesis is but a combination of the other two kinds of change in which both or all of the substances used are decomposed and the products combined in a new way.

15. Mixtures.-A mixture differs from a compound in that the composing substances do not unite with each other as in additive reactions; neither is there any rearranging of the elements into new groupings. Further, as a rule, no definite amounts of the two substances are used, or at least are not necessary. The particles of one intermingle with those of the other, but each retains all its own properties. White sand may be mixed with common salt, but neither has lost its distinguishing properties. A little placed on the tongue will possess a salty taste and at the same time will have the gritty feeling of the sand. Moreover, one may readily be separated from the other

by adding water, stirring and after a few minutes decanting or filtering. A common case given by nearly all books is that of fine iron filings and flowers of sulphur. Mixed together, the result is a greenish-gray powder, but neither has lost its distinguishing properties, and as in the case of the salt and sand they may be readily separated. With patience most of the iron filings may be successfully separated from the sulphur by a good magnet. An easier and more satisfactory method is to add carbon disulphide which upon shaking will dissolve the sulphur as water will salt. The dissolved sulphur may then be poured off through a filter paper; if the filings are washed with another portion of the carbon disulphide all the sulphur may be removed. By evaporating the liquid the sulphur may be recovered with all the properties it possessed before. If, however, the intimate mixture be heated for some time strongly in a test tube, the resulting mass will be black instead of greenish, will not be attracted by the magnet as the filings were, and the sulphur cannot be removed by solution. Chemical union has taken place and we now have a compound of iron and sulphur, called iron sulphide.

Exercises for Review

1. Do you believe Aristotle's statement about the globe of sand? Give reason for your answer.

Can

2. At the present time how is scientific truth obtained? reason aid at all in the discovery of Nature's laws? Explain. 3. What was the old idea regarding the relation of water to air? What facts had they to cause such a belief?

4. What is meant by transmutation? What led the ancient philosophers to believe in such a thing? Do you believe it possible? Why?

5. What is matter? How many kinds can you find in this book? In this room? Give two old theories about the composition of

matter.

6. What was Boyle's idea of matter? What is the present idea? Is this necessarily the true idea? Explain.

7. Define an element. How many are known? How many are liquids? Solids? How many constitute nearly the whole of the

earth? What two are the most abundant?

8. What is à compound? Name two and give composition. Explain what is meant by chemical union. Illustrate.

9. What is the general plan of naming compounds? Give the signification of the endings ide and ate. Give illustrations.

10. Into what two classes are elements divided? Will copper and silver unite to form a compound? Give reason for your answer. 11. How can one learn experimentally whether an element belongs to one class or the other? Give meaning of the terms anode and cathode. What synonyms are sometimes used?

12. Name three kinds of chemical changes. Give illustration of each. Show how the third may be regarded as a combination of the other two.

13. How does a mixture differ from a compound? Name two mixtures and give some easy way of separating them.

Outline

CHAPTER II

WATER AND HYDROGEN PEROXIDE

Forms of Appearance

Characteristics of Pure Water

Water in the Human Body and Foods

Necessity of Water to the Body

Proof of Composition

(a) By Electrolysis

(b) By Weight

Law of Definite Proportions
Water of Combination

Hydrates

Efflorescence

Deliquescence

Domestic Water Supplies

City Water Supplies

Hydrogen Peroxide

Law of Multiple Proportions

1. Its Familiarity. Water is at once the most familiar of all natural substances and one of the most interesting. It appears in a very large variety of forms, all more or less familiar: in partially condensed vapor as fog; in the feathery cirrus cloud; the billowy cumulus, the beautiful summer cloud; the stormy and threatening nimbus. In the solid form as snow, hail, ice, glaciers and icebergs.

2. Characteristics of Water.-Pure water is tasteless, odorless and colorless, except in great depths as lakes and seas, when it appears blue or bluish green. It is often said that distilled water tastes flat, but this is because we are accustomed to drinking water somewhat impure. Just as beans or potatoes or other vegetables served without salt would taste flat so does water with

out the usual impurities. Pure water when evaporated leaves no residue, hence would form no incrustation on the inside of kettles or boilers.

3. Water in the Human Body.-Not only does water appear in such variety of form and such quantities in nature, but it constitutes a very large proportion of the animal body. Only about 40 per cent of the human body is solid matter, while in the lower animals the percentage of water is much higher. Our foods are also high in water content. Even butter and flour, which we often think of as dry, contain as much as 12 to 14 per cent of water for the former, and 10 to 11 for the latter. The following table will give an idea as to many of the common food products: