摘要：

This review covers 106 years of experimental work. Hardly an item has been mentioned for which there have not been opposing data to be considered. This fact has required a rather formidable length of presentation of data. Every effort has been made to give opposing views fairly. But where it is possible to do so, conclusions must be drawn if progress is to be made. The reviewer has set down the conclusions he believes are warranted by the data, but references to the text and, in the text, numerous references to original sources are given, so that the reader can consult the basis for the conclusions stated and arrive at better ones if he can.

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