EDITOR'S PREFACE. 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 except for the correction of a few trifling misprints. I have thus endeavoured to bring into clear prominence at the outset the scientific value of the light the book sheds on the constitution of matter. The world owes a debt to scientific men of the ordinary type that cannot be over-estimated, but though they have hitherto preferred to progress gradually, from point to point, disliking leaps in the dark, the leap now made is only in the dark for those who will not realise that the progress to be accomplished by means of instrumental research must sooner or later be supplemented by subtler methods. Physical science has reached the conception that the atoms of the bodies hitherto called the chemical elements are each composed of minor atoms. Instrumental research cannot determine by how many, in each case. Occult research ascertained the actual number in some cases by direct observation and then discovered the law governing the numbers in all cases, and the relation of these numbers to atomic weights. The law thus unveiled is a demonstration of the accuracy of the first direct observations, and this principle once established the credibility of accounts now given as to the arrangement of minor atoms in the molecules of the numerous elements examined, seems to me advanced to a degree approximating to proof. It remains to be seen-not how far, but rather how soon the scientific world