When undertaking to prepare a new edition of this book I received permission from the authors to "throw it into the form in which you think it would be most useful at the present time." It was left to my discretion, "What to use and what to omit." I have not found it necessary to avail myself to any considerable extent of this latter permission. But as the contents of the book were originally arranged the reader was ill-prepared to appreciate the importance of the later research for want of introductory matter explaining how it began, and how the early research led up to the later investigation. I have therefore contributed an entirely new preliminary chapter which will, I hope, help the reader to realise the credibility of the results attained when the molecular forms and constitution of the numerous bodies examined were definitely observed. I have not attempted to revise the records of the later research in which I had no personal share, so from the beginning of Chapter III to the end the book in its present form is simply a reprint of the original edition