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must it include recent physico-chemical conceptions, such as equilibrium, that have a general application to both physical and chemical changes.

The revision of the “ Essentials” has been made with the purpose of meeting these demands. The most important changes and additions are the following:

1. The theoretical matter has been presented earlier than in the original edition, and it has been broken up into portions more suitable for study and assimilation. Chapter I contains, besides the specifically chemical topics of changes, elements, etc., a short account of matter, energy, mass, and density. The idea that energy is associated with matter in substances is brought out in Chapter II, that on Oxygen. The gas laws are given in Chapter III. Problems on the relation of volume to temperature and pressure are solved by the fractional method rather than by the method of proportions. The laws of definite proportions and of equivalent weights are given in the chapter on Water (V). The law of multiple proportions and the atomic and molecular hypotheses appear in Chapter VII. Chapter VIII treats of equations and of calculations from formulas and equations. The methods for determining molecular weights are given in Chapter XII; those for atomic weights, in Chapter XIII. Molecular formulas and equations, constitutional formulas, and isomers are topics of Chapter XVIII.

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2. The treatment of Valence, which took only a part of a chapter in the first edition, makes up Chapter XI of the revised book. A valence table of the common elements and radicals is given, and the application of valence to the construction of formulas receives special attention.

3. Equilibrium, like valence, has been given a separate chapter. The notions of physical equilibrium, as applied to changes of state and to solution, precede the discussion of chemical and ionic equilibrium.

4. The influence of the several factors in solution and crystallization is brought out in Chapter VI, and several solubility curves are given. A quantitative table of solubility for many common compounds is inserted in the Appendix.

5. The chapter on Acids, Bases, and Salts (XIV) contains a section on equivalent (normal) solutions. The theory of ionization and electrolysis is given in Chapter XV.

6. The “nascent state” is ascribed, not to the atomic state of the nascent element, but to the presence of catalyzers or excessive amounts of energy.

7. The new book is especially strong in its industrial features. The following facts illustrate this:

a. The methods of purifying and softening water are given in detail.

b. The commercial methods of making hydrochloric and nitric acids receive adequate attention and special illustrations, as does the “contact process ” for sulphuric acid.

c. The applications of the electric furnace and modern electrolytic processes are carefully considered.

d. The description of the copper, iron, and steel manufacture is detailed and thoroughly modern.

e. Mordants and dyes are treated in a fundamental way.

8. The new book shows that the properties of the sulphides form the basis of the Qualitative Analysis of the metals, and the Qualitative grouping is compared with the Periodic Arrangement.

9. The Exercises of the revised text are largely new, and more numerous than in the original edition.

10. The new book has 92 illustrations, as compared with 65 in the older edition. All but 10 of these are from new drawings. Every page of the book has been reset.

The reviser wishes to express his thanks to Dr. Oliver C. Farrington, of the Field Museum of Natural History, Chicago, for the photograph of White's Cave (Fig. 64); to the J. T. Baker Chemical Company, publishers of The Chemist-Analyst, for the illustrations of the hydrochloric acid and nitric acid manufacture (Figs. 32 and 50); and to the United Gas Improvement Company, Philadelphia, for the cut illustrating the making of “ water gas" (Fig. 68).

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About half of the drawings for the new illustrations were made by Miss Margaret C. Hessler. The remainder are the work of Mr. Corry Wilkin, Mr. Barton Westervelt, Mr. Andrew Bolay, and Mr. Claude Postlewait. To all of these the reviser expresses his thanks. He is also greatly indebted, and correspondingly grateful, to his colleague, Dr. Eugene C. Woodruff, of the James Millikin University, and to Mr. Leslie A. Touzalin, of the Illinois Steel Works, Chicago, for many helpful suggestions given during the preparation of this revision.

J. C. H. JUNE, 1912.

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