xiv LIST OF ILLUSTRATIONS 



PAGE 



to a large extent. In any case, in view of the apprehended exhaus- 

 tion of the world's petroleum fields in the near future, these shale 

 deposits constitute a most important national asset. 



PHOTOMICROGRAPHS SHOWING THE STRUCTURE OF STEEL MADE BY 



PROFESSOR E. G. MAHIN OF PURDUE UNIVERSITY 490 



1. Cold-worked steel showing ferrite and sorbite (enlarged 500 

 times). 



2. Steel showing pearlite crystals (enlarged 500 times). 



3. Structure characteristic of air-cooled steel (enlarged 50 times). 



4. The triangular structure characteristic of cast steel showing 

 ferrite and pearlite (enlarged 50 times). 



REPAIRING THE BROKEN STERN-POST OF THE U. S. S. " NORTHERN PA- 

 CIFIC ": THE BIGGEST MARINE WELD IN THE WORLD .491 



On the right the fractured stern-post is shown. On the left it is 

 being mended by means of thermite (see page 468). Two crucibles 

 each containing 700 pounds of the thermite mixture are seen on the 

 sides of the vessel. From the bottom of these the melted steel 

 flowed down to fill the fracture. 



MADAME CURIE IN HER LABORATORY 542 



Marie Sklodovska, a Polish refugee stranded in Paris, was first 

 engaged in the physical science department of the Sorbonne to 

 wash bottles and prepare the furnace. Professor Lippmann, the 

 pioneer in color photography, promoted her to setting up apparatus, 

 and put her to do work with Pierre Curie, one of his assistants. In 

 1895 she became Madame Curie. 



In 1903 the results of Madame Curie's work on radium were pre- 

 sented to the faculty as a thesis for the doctor's degree. The thesis, 

 unlike that of Arrhenius, was favorably received, and shortly after- 

 wards the Nobel Prize in Physics was divided between the Curies 

 and Becquerel, whose previous work on uranium had suggested the 

 research. 

 In 1911 Madame Curie was awarded the Nobel Prize in Chemistry. 



FOG-TRACKS FROM RADIUM 543 



The positively - charged helium atoms (alpha-particles), thrown 

 off from radium, in passing through the air ionize the molecules 

 with which they collide, and these ionized molecules have the same 

 power that dust possesses (see p. 293) of affording nuclei on which 

 moisture may condense. Hence, when a particle of a radium 

 compound is supported in a flask containing air saturated with 



