78 



DISPERSE SYSTEMS 



TABLE XI 



Corresponding Absorbed and Subjective Colours 



{a) Wave-lengtli in ju. 

 Absorbed Colour 



(h) Transmitted Colour 



Wave-length in fx 



•70 

 Purple 



Green 



•50 



•65 

 Red 



•60 

 Orange 



Blue 



Green- 

 Blue i 



•48 ^45 



■55 

 Yellow 



Indigo 



•43 



•53 

 Green- 

 Yellow 

 Violet 



•40 



light of longer wave-length, e.g. from blue or green, through various 

 shades of yellow and orange to red. If the suspended particles 

 are very fine, blue light is, as we have noted above, scattered 

 laterally, while red light is transmitted. Such a system will 

 appear red by transmitted light and blue by reflected light {e.g. 

 skim-milk, tobacco smoke and colloidal gold). It has also been 

 shown that as the particles decrease in size, the absorption bands 

 in the spectrum of the solution shift towards the ultraviolet 

 (Ostwald). 



Opticai, Resonance. — The amplitude of vibration of a particle 

 is a function of its mass, temperature being kept constant. As the 

 mass alters so will the period of vibration. According to Wood, 

 metallic particles, if highly dispersed, owe their colour not to 

 ordinary reflection, diffraction, interference, etc., but to optical 

 resonance. Resonance is the production of vibrations in a body 

 by the periodic application of a stimulus which has the same period 

 as the natural period of the body. The vibrations of a tuning fork 

 may be transmitted through the air and cause to vibrate another 

 tuning fork of the same pitch. Since the resonator owes the energy 

 necessary to set it into vibration to the stimulating body, it follows 

 that the stimulating body must lose energy to the resonator. 

 The particles in colloidal solution are supposed to be vibrating 

 with the same frequency as light of a certain wave-length. Con- 

 sequently, they will receive energy from the light which will tend 

 to increase their amplitude of vibration. The kinetic energy 

 of the solution will tend to increase, but any increase in kinetic 

 energy would mean increase in tcmpcratui'e and a slight alteration 



