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to the processes of fermentation, and the comprehensive group of alcohols, and their derivatives, the ethers and allied compounds.

'In treating these various classes of compounds, the author preferred to examine successively the different members of each homologous group, before passing to the consideration of the derivatives from the typical or leading member of each group. For instance, in the case of ordinary alcohol, instead of describing ether, aldehyd, and acetic acid in succession,—the different varieties of alcohol, such as wood spirit, fousel oil, &c., are first described ; then the series of the vinic acids, and then that of the ethers. By thus presenting the different members of the same homologous series in succession to the student (see p. 40) he is enabled to trace more readily their similarities and their differences, and to note the general method adopted in their preparation, than if his attention were distracted by passing to other bodies of totally different character and properties. This plan conduces to brevity as well as to clearness, and facilitates the description of the various series from a more general point of view, than would be readily practicable, if the arrangement followed by most writers since the appearance of Liebig's classical work on organic chemistry

The most important changes in the present edition consist in the adoption of a new form of notation, and in a recurrence in the concluding volume to the system of nomenclature introduced by Berzelius. This nomenclature has been used occasionally ever since it was proposed, but it was not extensively adopted by chemical writers in this country until its merits were prominently insisted upon two or three years ago by Professor Williamson.

The substitution of such terms as potassic nitrate for nitrate of potash, sodic sulphate for sulphate of soda, and zincic oxide for oxide of zinc, may, like most changes, appear at first to some extent awkward and pedantic. Yet the system possesses advantages in brevity and precision which will, there can be little doubt, lead to its gradual substitution for the older nomenclature.

The change in notation will doubtless be attended with more inconvenience; though it is certain to be adopted, since in none of the recent investigations made in this country, and in very few of those on the Continent, is the old method made use of.

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It is remarkable that the ratios expressive of atomic weights in the symbols employed by Berzelius, at their first introduction by him nearly fifty years ago, should be, with few exceptions, those to which Cannizzaro and Wurtz with other writers of the present day have returned.

In order to facilitate the use of this new notation, a table of the symbols and atomic weights adopted in the present volu me has been prefixed. In a few instances of the more common bodies of the organic kingdom, the old formula has been given by the side of that now preferred. Those symbols which, in consequence of the alteration, have become changed in value, are indicated in the new formulæ by barred letters. It is easy to transform the new values into those formerly adopted by doubling the numbers attached to each of the barred letters, leaving the others unaltered : alcohol, for example, may be represented either as 6,4,9, or as C,H,02.

King's COLLEGE, LONDON,

October, 1866.

TABLE OF CONTENTS.

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