PREFACE XV 



The Deutsche Museum in Munich has provided us with 

 Figures Nos. 4, 25, 27, 29, 37, 41, 57, 62. 



From the Corpus Imaginum of the Photographische 

 Gesellschaft I have taken the following figures: -6, 11, 16, 

 19, 26, 30, 31, 33, 39, 40, 47, 50, 52, 53, 55, 61. 



Figure 32, Davy (page 194), is taken from an engraving, 

 after a painting by Sir Thomas Lawrence, found in Davy's 

 Reminiscences y published by his brother. Figure 46, 

 Tyndall (page 282) is from a drawing by George Richmond. 

 Figure 43, Faraday, with the 'heavy glass' (page 262) is 

 from the book Life and Letters of Michael Faraday by 

 Bence Jones. Figure 44, Weber (page 263) comes from 

 Zollner's memorial of Weber. Figure 45, Mayer (page 282), 

 is from Weyrauch's Centenary volume to the memory of 

 this scientist. 



My thanks are also due to friends and colleagues, who 

 kindly assisted me in collecting data and portraits, and 

 also in advance to those who will help me further as I 

 hope in the future to improve the work. 



P. L. 



Heidelberg 



