Xll PREFACE. 



Weinhold's own Preface, with which I will bring niy 

 remarks to a close : 



' The most indispensable qualification for anyone who 

 wants to make real use of this book is perseverance. 

 Skilfulness in practical operations, such as the working 

 of metals or of glass, or in actually making experi- 

 ments, is only to be attained through practice ; and 

 since it is impossible for even the most careful 

 written instructions to provide against every mistake 

 that a student can possibly make, personal experience 

 must every now and then be the real teacher. When-, 

 ever anything does not succeed the first time, the 

 student should try it again ; but he should not try 

 thoughtlessly, on the mere chance of better luck next 

 time : he should endeavour by careful consideration to 

 find out the cause of his ill success. The completion 

 of a piece of apparatus, or the success of an experi- 

 ment, well repays the trouble spent upon it ; the skill 

 that has been gained in the process never turns out 

 useless afterwards, and occasions arise on which even 

 those whose chief occupations lie in quite other direc- 

 tions are able to recognise its value.' 



G. C. FOSTER. 



UNIVERSITY COLLEGE, LONDON : 

 December 1874. 



