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A special feature of the work is the careful comparison between theory and experiment at every stage. From our standpoint as to the aim of science, such a comparison appears all-important. The one difficulty in this matter has been to get access to the most accurate and recent experimental results; a difficulty arising entirely from the present isolation of the writer. At the ends of the earth there are no great libraries to which to refer, and the procuring of material from 'home' is tedious and uncertain. In dealing with experimental results I have deliberately refrained from entering upon descriptions of the methods of experiment as, in my judgment, quite out of place in such a work as this.