. 



vi AUTHOR'S PREFACE TO THIS EDITION 



to develop the theory by calculation, but rather to support 

 it by observation and found it on experiment. I therefore 

 collected together, as completely as I could, and summarised, 

 the observations by which the admissibility of the theory 

 might be tested and its correctness proved. I dare to think 

 that I have not failed in this endeavour ; for not only did my 

 book receive favourable judgment, but also my presentation 

 of the subject was turned to much account in popular 

 writings and found its way into physical text-books. 



In this my book fulfilled the purpose I intended. I 

 wrote it only for that time, and did not seriously think that 

 it might reappear in a new edition after long years. But 

 the demand for the book did not cease, and, as the supply 

 was nearly exhausted more than ten years ago, I had to 

 determine on a revision, although the necessary preliminary 

 work had not yet been done. 



As to the general plan of the revision I could not long 

 be in doubt. I could not destroy the shape in which the 

 book had been proved and had won recognition, and I have 

 therefore written it anew on the old plan. The mathematical 

 discussions form, as before, an Appendix which makes no 

 claim to completeness and need not be studied by every 

 reader; the contents of the book are limited to the same 

 range of phenomena as in the first edition ; the observations 

 which I have taken into consideration belong partly to 

 mechanical physics and partly to the domains of heat and 

 chemistry. The book has therefore preserved its character, 

 and may perhaps also serve its first purpose. 



I have inserted a series of new theories, as, for instance, 

 an explanation on the kinetic theory of the resistance of air 

 and of the reaction of a jet, together with a longer investi- 

 gation, also new, concerning the influence of the dissociation 

 of the molecules of a vapour upon its viscosity. I should 

 have enlarged the contents of the book in many other 

 directions with pleasure ; I should gladly, for instance, have 



