COLLOIDAL METALS. 



51 



liquid glass. The sulphuric acid is more transparent than water, while 

 the didymium nitrate has a sharp absorption band at 0.76 ft. The latter 



90% 



1.7, U 



Fig. 33. Potassium alum (a); Borax solution (b); 

 Water (c); Potassium permanganate solution. 



Fig. 34. Sulphuric acid (a); Lanthanum nitrate (6); 

 Didymium nitrate (c); Liquid glass. 



have absorption bands in the visible, so that they would not be useful for 

 efficiency work. The same is true of liquid glass. 



Chlorides of the Yttrium Group (Sc.Yt, La,Yb); Neodymium Nitrate [Nd(N0 3 )3]. 

 (Cell i cm. Concentration unknown. Fig. 35.) 



These solutions, like the preceding ones, were obtained from the Chem- 

 ical Laboratory of Cornell University, and the concentration was unknown. 

 They were examined to learn whether the sharp absorption bands in the 

 visible are also to be found in the infra-red. It will be noticed that the 

 didymium nitrate (curve b, fig. 35) has two bands at 0.78 and 0.98 p, 

 while the yttrium group of chlorides have a band in common with didym- 

 ium at 0.98 pt. 



GROUP III: TRANSMISSION SPECTRA OF COLLOIDAL METALS. 



The metals and non-metals present two distinct types of absorption, of 

 reflection and of emission spectra. The metals (electrical conductors), even 

 in very thin films, are extremely opaque to all radiations throughout the 

 spectrum, except gold, silver, and copper, which have narrow transparent 

 bands at 0.32, 0.5, and 0.6 p, respectively. 1 The opacity is really due to 



1 Hagen & Rubens: Ann. der Phys., 8, p. 432, 1902; Javal: Ann. de Chim. et Phys. (8), 

 4, P- 137, 1895. 



