viii PREFACE 



been made to avoid severely adverse or destructive criticism 

 and finality in statement, for both are out of place in science, 

 especially today, when change and doubt are the very spirit of 

 scientific thought, when the "immutable" elements are known 

 to be mutable, when constancy in size and weight is no longer a 

 necessary quality of the molecule, and when the laws of thermo- 

 dynamics are being questioned. Laws and definitions should be 

 clear and precise but not final; otherwise we fall into the dogma 

 which dominated the science of the last century. 



Needless to say, a volume such as the present one is the work 

 of one man only in so far as he has penned it. The information 

 given on the following pages is the result of the work of a large 

 number of investigators. Many of them are mentioned, but 

 many more are omitted. To name only the outstanding workers 

 in the numerous and diverse fields of endeavor discussed in this 

 book would be a futile task and would defeat the purpose of the 

 book. An encyclopedic account of the subject is not our present 

 purpose. The author has mentioned those investigators whose 

 work he knows best, fully reahzing that another author would 

 make a different selection. After all, names add but a human 

 touch, always delightful in any historical account but never 

 important in the advancement of knowledge. 



Writing about the experiments of others is a very pleasant task 

 when one has the assistance of the workers themselves. The 

 richest possession of a scientist is the sympathetic interest of 

 his colleagues. Such an interest has made this book possible. 

 Were full acknowledgment given to all for help received, the 

 mere Ust of names would defeat its own purpose. Several 

 of my coworkers have, however, given such substantial assistance 

 that I wish to single them out for an expression of gratitude, 

 without being unmindful of the many others to whom I owe 

 much. I am particularly grateful to Professor Herbert Freund- 

 hch, who has read a major part of the manuscript of this book 

 and contributed much to accuracy in statement, especially in 

 those chapters dealing with strictly colloidal phenomena. To 

 Professor Howard PuUing and Dr. Laurence Moyer I am greatly 

 indebted for the reading and criticizing of a number of 

 chapters. 



Philadelphia, Pa., William Seifriz. 



December, 1935. 



