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TO THE SIXTH EDITION.
TN the present Edition, short notices of some of the most recent discoveries in those branches of Physics which are related to Chemistry have been incorporated in the work, and several new paragraphs have also been introduced, especially in the section on Polarization of Light, a subject which is becoming of great importance to the chemist in the study of many organic compounds.
An attempt has been made to bring the terminology into accord with the views of the present time, and in this the Editor has received much valuable assistance from his friend and former colleague, Mr. A. G. Greenhill, Professor of Applied Mathematics to the Advanced Class of the Royal Artillery Institution. Dr. Andrews has also very kindly revised and corrected that portion of the book which contains an account of his researches on the Heat of Combination and the Condensation of Gases.
Nearly all the references have been verified, and the year of publication introduced; it has also been found advisable to recalculate all numbers which were capable of such treatment, and thus many inaccuracies have been discovered and removed.
Cooper's Hill, August, 1877.
TO THE FOURTH EDITION.
T^HE principal changes in this Edition consist in the adoption of the new system of Atomic Weights, with the method of notation dependent upon it; and the introduction of the Centigrade values for degrees of temperature, in addition to those on Fahrenheit's Scale, as well as of the Metrical Equivalents to the English weights and measures, employed in the description of processes and apparatus.
The use of barred letters to denote the symbols applied to the new atomic weights has been discontinued, as it is no longer necessary, inasmuch as the new values alone are employed in the present Edition.
There may be considerable difference of opinion as to
the expediency of endeavouring to introduce the metrical
system of weights and measures into the transactions of
commerce, and the concerns of daily life. But there is a
general and increasing desire amongst men of science to
secure the adoption of some uniform system in the scientific
writings of men of all countries. It would not be difficult
to show that the metrical system has its defects. But it
is founded upon principles that are simple, intelligible, and
consecutive, and it is capable of convenient application.
Moreover, and this is of special importance, it is widely
adopted in various countries of Europe, as well as in North
America, and the number of scientific men who employ it