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processes we trace in the chemistry of the atmosphere. In this indirect but natural way, we have introduced as much of organic chemistry as could well find place in an elementary manual.

The metals we have grouped more according to their uses than according to their chemical relations, with a view to make the subject less dry and more practical.

It is difficult to write a book suited to the wants of different schools, or even of successive classes in the same school; since the schools vary in grade, and the classes in ability. We have attempted to meet this difficulty by putting into the body of the book the simpler and more practical portions of the subject, which can be readily mastered by young pupils of average ability, and by adding in the Appendix a concise statement of the elements of chemical philosophy as now understood. In this way, the simpler and the more difficult parts, instead of being confusedly mixed together, distinguished merely by the common and clumsy expedient of coarse and fine print, are treated separately, so that each may be complete and symmetrical. At the same time, the two parts do not overlap, but form successive chapters progressively arranged. Qur experience as teachers has convinced us, that this is, on the whole, the best method of meeting the wants of successive classes ; and the marked favor with which our " Handbook of the Stars," an elementary Astronomy” on precisely the same

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plan, has been received, shows that the experience of many of our fellow-teachers has been pretty much the same as our own.

We have not forgotten that the theories of chemistry have been revolutionized within a few years ; and we have of course used the new symbols and notation, without which it is as impossible to teach modern chemistry as it would be to teach modern astronomy in the language of the old astrologers. Until within a year or two, there was some excuse for clinging to the old notation, as rival systems were contending for the supremacy. But even as long ago as October, 1866, Miller, the first English authority on the subject, wrote as follows: "The change in notation is certain to be adopted, since in none of the recent investigations in this country, and in very few of those on the Continent, is the old method made use of.” Since that time, the system of symbols and notation used by us in this book has been formally adopted by the Chemical Society of London, whose decision is recognized as law in England; and it is now taught in Harvard College, which will certainly be acknowledged as one of the highest authorities in this country.

In the chapter on the Chemistry of the Atmosphere, we have been especially indebted to the Lectures on “Religion and Chemistry” (New York, 1866), by Professor Josiah P. Cooke, Jr., of Harvard College; and the chapter on Chemical Philosophy is almost wholly drawn from the "First Principles

of Chemical Philosophy” (Cambridge, 1868), by the same eminent author. His remarkable power of presenting difficult subjects in a way at once clear and attractive, makes these books especially valuable to the teacher.

We have been under obligations, also, to Miller's " Elements of Chemistry” (London, 1866), and Roscoe's " Elementary Chemistry” (London, 1866). These books (both of which, we are glad to see, have been recently reprinted here) as well as Hofmann's " Modern Chemistry” (London, 1865), and Wurtz's " Introduction to Chemical Philosophy” (London, 1867), the teacher will find useful for purposes of reference.

CAMBRIDGE, December 10, 1868.

Teachers will observe that, except in the Appendix, either the old chemical name or the ordinary commercial name of the substance is generally added, in parentheses, to the new chemical name.

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