i873«l Colours and their Relations. 83 



and capable of exciting the optic nerve. The case resembles 

 that of a bass string set a vibrating by the vibrations of a 

 treble string several octaves higher in the scale. 



This phenomenon affords evidence that the molecules 

 of bodies are actually made to vibrate by the ethereal 

 waves, and do in their turn propagate a secondary set of 

 ethereal vibrations — so far favouring the second view of the 

 nature of intrinsic colours. Indeed, according to this view, 

 the only difference between fluorescent colours and ordi- 

 nary intrinsic colours consists in this circumstance, that, 

 whereas the latter are due to vibrations established in the 

 molecules by ethereal waves lying within the limits of the 

 visible spectrum, the vibrations causing fluorescence are 

 established by ethereal vibrations more rapid — sometimes 

 greatly more rapid than themselves. If the incident light 

 be winnowed from all waves of shorter period than the 

 green, there is no fluorescence ; and in this sense the fluor- 

 escent tint may be regarded as adventitious ; because it 

 depends for its exhibition on the character of the incident 

 light. But it is in another aspect intrinsic ; because it 

 depends on the molecular vibrations of the fluorescent body. 



The flame of a spirit-lamp, though deficient in light capable 

 of stimulating bodies to exhibit their intrinsic colours, 

 abounds in that sort of light which stimulates fluorescent 

 bodies. If in a dark room a spirit-lamp be lighted and 

 placed behind the observer, and if he put on a smooth black 

 surface a drop of water, and alongside of it a drop of a weak 

 solution of the disulphide of quinine, or of the bark of the 

 horse-chesnut, and examine these by placing them near the 

 level of the eye, while the drop of water will be hardly 

 visible, that of the fluorescent liquid will appear quite solid 

 and of the colour of a turquois. 



Another phenomenon, illustrating the great influence of 

 the molecular condition of bodies upon the light falling on 

 them, is that of temporary colours. The most familiar 

 example of these is furnished by the sympathetic inks 

 formed by the chlorides of cobalt and nickel. Very dilute 

 solutions of those salts are so nearly colourless that when 

 laid on paper they are invisible. But when subjected to 

 heat the former becomes blue, the latter yellow, while by 

 combining the two a green is obtained. These colours 

 gradually disappear when the paper stained with them is 

 exposed to the air,; but they may be restored again and 

 again by mere warmth. The explanation is that the heat 

 drives off all moisture from the salts, and their molecules 

 when dry tend to vibrate — the one in unison with the blue, 



