xii PREFACE 



importance of their work, according to the degree of inward 

 and outward difficulty which, in view of the circumstances 

 of their times, they had to overcome, and finally also, ac- 

 cording to the signs of intellectual greatness which we can 

 recognise from the general character of their personalities. 

 Important work of a few other investigators, or of persons in 

 some way concerned in an investigation, is dealt with besides, 

 and a full index of names will be found at the end of the book. 



Particular mention must be made of the limitation of the 

 matter as regards its extension to the present day. It is 

 obvious that living persons had to be omitted; but even 

 outside this limitation, a certain historical perspective was 

 necessary, if success was to be attained in presenting a series 

 of investigators judged as uniformly as possible. I decided 

 to take the War as the limit of time, so that no scientist 

 would be dealt with who had survived it. This condition 

 also turned out to form a possible basis, without unduly 

 limiting the range of the material treated, and excluding 

 entirely recent developments of our knowledge. This 

 recent development, quite in accordance with its origin, 

 could be touched upon in connection with the work of the 

 latest investigators considered, to such an extent that no 

 very important part of our whole picture of nature remains 

 without reference. Two exceptions had, however, to be 

 made as regards the limitation in time: Van der Waals and 

 Crookes, whose deaths, on account of the unusually great 

 age to which they lived, came after the end of the war. 

 It was impossible to omit them, since they were contem- 

 poraries of other investigators whose work fell within the 

 chosen limit of time. 



The great investigators, whose works I had always 

 admired, appeared to me, ever since my acquaintance with 

 their lives, as both deserving of reverence, and supreme in 

 their greatness; but none the less, at first dimly and later 

 more clearly, I came to feel a kinship with them, and this 



