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In conclusion, the author's thanks are due to the University of Glasgow for. providing facilities for carrying out the research, and to the Carnegie Trust for the Universities of Scotland for the loan of several instruments employed in the tests. The author is also indebted to Mr. A. L. Tackley, B.Sc, for the great interest that he has taken in the experiments, and to two of the students, Mr. Norman F. B. Begg, B.Sc, and Mr. B. N. Das, B.A., B.Sc, for assistance with several of the experiments.

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