418 Dynamic Theory. 



organization is due exclusively to light. We have seen that food may 

 have something to do with it, and we are certain that temperature also 

 has much to do with it, or at any rate radiant heat has. But there are 

 abundant proofs that light stimulates the deposition of pigment matter 

 and that there is a mutual action and reaction between the two. The 

 probability is that the various sorts of pigments are influenced by va- 

 rious tones of radiant energy, both heat and light, and also by sound, by 

 food and by the general vital energy of the organism. From what has 

 been said of the colors of bodies we conclude that they depend exclu- 

 sively on the molecular constitution and arrangement of the bodies and, 

 therefore, whatever tends to alter this molecular constitution may change 

 the color. As light among other agencies does this or contributes to- 

 wards it, certain of the organic pigments must be regarded as subject to 

 its influence. On the other hand no sooner does the new molecular 

 change take place or the new pigment form than it begins to operate as 

 a modifying agency upon the light It becomes a barrier to shut it off, 

 to absorb it and reduce it to heat, or a filter to transmit rays of a certain 

 tone while it refracts those of another. The ways in which the deposi- 

 tion of pigments is determined by the action of light, may be, and 

 doubtless are, various. First, and principal, is the alteration of the molec- 

 ular constitution, which without change of the constituent elements 

 gives them a different refractive power on the light waves. This is a 

 fact similar to that concerning the parallel effect of heat giving a new 

 isomeric form and refractive power to the body without chemical altera- 

 tion. Some examples of this effect of heat may be mentioned. Mer- 

 curic oxide, which is orange yellow, becomes orange red and brown 

 when heated ; chromic oxide is green, and when heated becomes } T el low; 

 cinnabar, which is scarlet, changes to puce ; metaborate of copper is 

 blue and changes when heated to green and greenish yellow. When 

 these substances are cooled back to the ordinary temperature they re- 

 sume their colors. Observing these changes to take place down the 

 scale from shorter to longer wave lengths, they seem naturally to connect 

 themselves with the expansion of the intermolecular spaces by the heat, 

 the more expanded they are (up to the point of self-radiation) the lower 

 the color. If, while under the influence of heat or of a similar action of 

 light, the incorporation of foreign mineral substances takes place, a per- 

 manent change of color is effected. The coloring matter in animals is 

 quite varied. Copper has been found in the red of the wing of the 

 Turaeo. Seven different coloring matters have been found in birds' eggs, 

 several of which are chemically related to those of blood and bile. The 

 same colors in different animals are not always produced by the same 

 substances, as proved by the fact that the red wing of the Burnet-moth is 

 changed to yellow by the action of muriatic acid, while the red of the 



