PBEFACE 



TO 



THE FOUKTH EDITION 



THE third edition of this book being exhausted, it was 

 thought advisable to make some revision and to write a 

 fresh Appendix, with a view to bringing it up to date. 



The Author was accordingly preparing to take the 

 work in hand, and had already discussed with me as to the 

 main points of the alterations, when, on December 16, 

 1905, his energetic and useful life was cut short by a 

 fatal railway accident at Saltford, in Somerset. 



Many and widespread have been the expressions of 

 sympathy with his family in their bereavement. From 

 literally all over the world have come letters testifying 

 to the respect in which he was held and to the loss 

 which workers in many branches of science feel at his 

 death. 



To me, his son and the companion and sharer in 

 much of his experimental work, the loss is not only that of 

 a revered and honoured father, but also that of a guide 

 and adviser in much of my own scientific work. 



a 



