104 PROTOPLASM 



solution changes from yellow to orange, rose, red, violet, blue, 

 purple, and black. The yellow color is not colloidal but ionic; 

 it is the color of a true molecular solution; i.e., it is not a structural 

 color, as are purple and blue. The color of the Tyndall cone 

 of gold dispersions changes from a faint, dull green to brilliant 

 green, yellow, and orange. When made in the laboratory, 

 colloidal gold may be red, purple, or blue ; but why is not known. 

 Small concentrations of electrolytes bring about different 

 colors. Distilled water ordinarily gives blue colloidal gold 

 when prepared by the Burton-Bredig electrical dispersion 

 method. A weak solution of potassium bromide or iodide, 

 for instance, will yield red colloidal gold. 



An interesting fact in colloidal color but one difficult of inter- 

 pretation is that nonmetallic, i.e., electrically nonconducting, 

 particles give different color effects from those given by metallic, 

 conducting particles. 



Occasionally, precipitates change color while forming; thus, 

 when Fehling's solution is added as a test for sugar, a red. precipi- 

 tate is the expected result. Frequently, however, an orange, 

 yellow, or yellowish-green precipitate is obtained. These color 

 changes appear to be associated with differences in the size of 

 the particle of the cuprous oxide formed. 



Bancroft and his colleague Mason have made an extensive 

 study of colloidal color, especially in birds and insects. They 

 state that blue eyes owe their color to a turbid medium which 

 is localized in the stroma. Increase in the size of the particles 

 of the turbid layer accounts for lessened clearness of the blue 

 with age. Pigmentation in the stroma may combine with the 

 colloidal blue to give green, hazel, or brown eyes. The non- 

 metallic blues in the feathers of birds such as the blue jay, the 

 bluebird, the indigo bunting, and the kingfisher are structural 

 (colloidal). No one has ever succeeded in extracting any blue 

 coloring matter from any blue feathers. The structural blue 

 is due to the scattering of light by minute air bubbles in the 

 horny mass of the barb of the feather. Green feathers result 

 when feathers which would be blue because of structure (pores) 

 are overlaid by a transparent layer of yellow pigment (carotin). 

 Metallic or iridescent colors, such as those of the peacock and 

 the humming bird, present a more difficult problem ; the question 

 is still open as to whether they are structural or pigment colors. 



