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XXXIV. On Mechanical Phosphorescence. 
Bij Andrew Stephenson*. 
1. TN explanation of the phenomena of phosphorescence it 
A is generally held that the energy communicated by 
the incident light is stored as potential energy of the mole- 
cules, and that on the removal of the light the change of 
configuration ceases to be stable and the system gradually 
returns to its initial state, the potential energy setting up 
vibratory motion, and thereby becoming dissipated as emitted 
light. The change occurring in the substance during this 
process is considered to be of such a radical nature as is 
designated by chemical change. 
We propose here to show that a simple type of mechanical 
system exhibits the chief phenomena of phosphorescence, and 
furthermore that the agreement between the properties of 
the phosphorescing substances and those of the system is of 
a quantitative nature. 
The energy stored by the system under the incident dis- 
turbance is kinetic, so that the explanation offered has nothing 
in common with that stated above. 
It will appear that the argument is general in character, 
applying to any mechanical system that can be treated by 
the method of normal coordinates. On account of the gain 
in vividness, however, it is convenient to make use of a 
particular system; the main facts forming the basis of the 
theory are then capable of simple exposition apart from the 
analytical development. 
2. Consider a particle suspended from a fixed point by a 
light elastic string ; if we regard the system as confined to 
a vertical plane it is fixed by two coordinates, and the normal 
motions are the vertical vibration and the horizontal (pen- 
dulum) swing. If the system is frictionless^a case which 
may be considered by way of introduction — the normal 
motions are in general practically independent ; the vertical 
vibration creates a small periodic change in the spring of 
the pendulum motion, while the latter exerts a second order 
forcing disturbance on the former, but neither of these 
actions is cumulative in effect unless the periods are 
properly adjusted. When the periods vertically and 
horizontally are approximately in the ratio 1/2 there is 
mutual interaction : if the system is initially in vertical 
motion, any small horizontal swing is gradually magnified 
through the periodic change of approximately double 
* Communicated bv the Author. 
