THE ACTIONS OF FORCES ON ORGANIC MATTER. 35 



matter : especially as the atom is held by a positive force be- 

 sides its inertia. The motion required can hence be given only 

 by successive waves; and that these may not destroy each 

 other's effects, it is needful that each shall strike the atom 

 just when it has completed the recoil produced by the impact 

 of previous ones. That is, the ethereal undulations must 

 coincide in rate with the oscillations of the atom, determined 

 by its inertia and the forces acting on it. It is also requisite 

 that the rate of oscillation of the atom to be detached, shall 

 differ from that of the atom with which it is united; since 

 if the two oscillated in unison the ethereal waves would not 

 tend to separate them. And, finally, the successive impacts 

 of the ethereal waves must be accumulated until the resulting 

 oscillations have become so wide in their sweep as greatly to 

 weaken the cohesion of the united atoms, at the same time 

 that they bring one of them within reach of other atoms with 

 which it will combine. In this way only does it seem possible 

 for such a force to produce such a transfer. More- 



over, while we are thus enabled to conceive how light may 

 work these molecular changes, we also gain an insight into 

 the method by which the insensible motions propagated to 

 us from the Sun, are treasured up in such ways as afterwards 

 to generate sensible motions. By the accumulation of in- 

 finitesimal impacts, atoms of ponderable matter are made to 

 oscillate. The quantity of motion which each of them even- 

 tually acquires, effects its transfer to a position of unstable 

 equilibrium, from which it can afterwards be readily dis- 

 lodged. And when so dislodged, along with other atoms 

 similarly and simultaneously affected, there is suddenly given 

 out all the motion which had been before impressed on it. 



Speculation aside, however, that which it concerns us to 

 notice is the broad fact that light is an all-important agent 

 of molecular changes in organic substances. It is not here 

 necessary for us to ascertain hotv light produces these com- 

 positions and decompositions. It is necessary only for us to 

 observe that it does produce them. That the characteristic 



