220 GOLD IN SCIENCE AND IN INDUSTRY. 



double power is well worth an effort. The mental stimulus, as well 

 as the new experiences garnered during the excursion, will sooner or 

 later react favorably on the practical problems, Avhile the earnest 

 wrestling with these problems may develop poAvers and intuitions 

 which will lend their own charm to the wider Droblems of science. 



GOLD AND SCIENCE. 



If we reperuse the table of the elements, not now in our capacity 

 as " plain men," but as chemists, we shall certainly not select gold as 

 of supreme interest chemically. Its position as chief among the 

 noble metals, its patent of nobility, is based on its aloofness from 

 common associations or attachments. ITnlike the element nitrogen, 

 it is mainly for itself and little, if at all, for its compounds, that gold 

 is interesting. In it we can at our leisure study the metal rather than 

 the elernent. Its color and transparence, its softness and its hard- 

 ness, the density as well as the extreme tenuity of some of its forms — ■ 

 such Avere the qualities which recommended it to Faraday when he 

 desired to study the action of material particles on light. I should 

 like to repeat to you in his own words the reasons he gave for this 

 choice : 



Because of its comparative opacity among bodies and yet possession of a real 

 transparency; because of its development of color liotli in tbe reflected and 

 transmitted rays; l)eeause of the state of tenuity and division wbicb it permitted 

 with the preservation of its integrity as a metallic body ; because of its supposed 

 simplicity of character, and because known phenomena appeared to indicate 

 that a mere variation in the size of its particles gave rise to a variety of result- 

 ant colors. Besides the waves of light are so large compared to the dimensions 

 of tbe i)articles of gold whicli in various conditions can be subjected to a ray, 

 that it seemed probable that the i)ni-ticles might come into effective relations to 

 the much smaller vibrations of the other particles. 



I may remind you that Faraday came to the conclusion that the 

 variety in the colors presented by gold under various conditions is 

 due to the size of its particles and their state of aggregation. Ruby 

 glass or ruby solutions he proved are not true solutions, nor are 

 they molecular diffusions of gold, but they contain the metal in aggre- 

 gates sufficiently large to give a sensible reflection under an incident 

 beam of light. Through the kindness of Sir Henry Roscoe I am 

 able to exhibit to you some of the original ruby-gold preparations 

 obtained during this research, which were afterwards presented to 

 him by Faraday at the Royal Institution some years before his death. 



By means of refined and ingenious optical methods Zigsimondy and 

 Siedentopf have succeeded in uiakiug these ultramicroscopic particles 

 visible in the microscope as diffraction disks; they have, further, 

 counted the number of particles per unit area and have from the 

 nitensity of their reflection calculated their size. In ruby glass 



