1 78 POPULAR LECTURES AND ADDRESSES. 



imperceptible, of a few atoms or molecules of 

 the attacking substance (oxygen, or iodine, or 

 sulphur, or chlorine, for example), and to go on in- 

 creasing until some such thickness as i/3O,oooth or 

 i/4O,oooth of a centimetre is reached by the film 

 of oxide or iodide, or whatever it may be that is 

 formed. 



The subject is one that deserves much more 

 of careful experimental work and measurement 

 than has hitherto been devoted to it. I allude to 

 it at present to point out to you how it is that 

 by this electric action we are enabled as it were 

 to sound the depth of the ocean of molecules 

 attracted to the metallic surface by the vapour or 

 gas entering into combination with it. 



When we come to thicknesses of considerably 

 less than a wave-length we find solid metals 

 becoming transparent. Through the kindness of 

 Prof. Dewar I am able to show you some 

 exceedingly thin films of measured thicknesses 

 of platinum, gold, and silver, placed on glass 

 plates. The platinum is of 1-9 X IO" 5 cms. thick- 

 ness, and is quite opaque ; but here is a gold film 



