THE ACTIONS OF FORCES ON ORGANIC MATTER. 33 



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 way 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 

 eventually acquires, effects its transfer to a position of un- 

 stable equilibrium, from which it can afterwards be readily 

 dislodged. 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 how light produces these compo- 

 sitions and decompositions : it is necessary only for us to 

 observe that it does produce them. That the characteristic 

 matter called chlorophyll, which gives the green colour to 

 leaves, makes its appearance whenever the blanched shoots of 

 plants are exposed to the sun ; that the petals of flowers, 

 uncoloured while in the bud, acquire their bright tints as 

 they unfold ; and that on the outer surfaces of animals, 

 analogous changes are induced; are wide inductions which 

 are enough for our present purpose. 



§ 14. We come next to the agency of chief importance 

 among those that work changes in organic matter ; namely, 

 chemical affinity. How readily vegetal and animal substances 

 are modified by other substances put in contact with them, 

 we see daily illustrated. Besides the many compounds which 

 cause the death of an organism into which they are put, we 

 have the much greater number of compounds which work 



