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 compositions 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 those milder effects termed medicinal--effects implying, like the others, molecular re-arrangements. Indeed, most soluble chemical compounds, natural and artificial, produce, when taken into the body, alterations that are more or less manifest in their results. After what was shown in the last chapter, it will be manifest that this extreme modifiability of organic matter by chemical agencies, is the chief cause of that active molecular re-arrangement which organisms, and especially animal organisms, display. In the two fundamental functions of nutrition and respiration, we have the means by which the supply of materials for this active molecular re-arrangement is maintained. The process of animal nutrition consists partly in the absorption of those complex substances which are thus highly capable of being chemically altered, and partly in the absorption of simpler substances capable of chemically altering them. The tissues always contain small quantities of alkaline and earthy salts, which enter the system in one form and are excreted in another. Though we do not know specifically the parts which these salts play, yet from their universal presence, and from the transformations which they undergo in the body, it may be safely inferred that their chemical affinities are instrumental in working some of the metamorphoses ever going on.