314 ON LIGHT. 



duced. In the disruption of one chemical combination, 

 and the constitution of another, a movement of mutual 

 approach, more or less direct, is communicated to the 

 uniting molecules, which, under the influence of enor- 

 mous coercive powers, is converted into a series of 

 tremulous, vibratory, or circulating movements com- 

 municated from them to the luminiferous et^er, and so 

 dispersed through space. Incandescence without com- 

 bustion (as in a piece of red-hot iron) must be looked 

 upon, from this point of view, as a result of the con- 

 tinuance of this vibratory movement after the primary 

 exciting cause has ceased, and of its gradual decay by 

 communication to the surrounding ether ; as a musical 

 string continues to sound after the blow which set it in 

 motion, till gradually brought to rest by the surround- 

 ing air. 



(99.) This may perhajos appear a digression from our 

 subject. But it will be recollected that our object in 

 these lectures is not to produce a treatise on optics, 

 but to fix attention on the immensity of the forces 

 in action, and the minuteness and delicacy of the me- 

 chanism which they animate in the most ordinary opera- 

 tions of Nature, and which the phaenomena of light have 

 been the means of revealing to us. We have no means, 

 indeed, of measuring the ^r/?/^/ intensity of the " coercive 

 forces" so called into action in the excitement of a lumi- 

 nous vibration, but that we are fully justified in applying 

 to them the epithet " enormous," the following consider- 

 ation will suffice to show. Whatever be the extreme 

 distance of excursion to which a vibrating molecule is 



