CHAPTER Y. 



COLORING MATTERS. 



THERE are found, in various parts of the animal body, a number of 

 substances which are distinguished by imparting to the tissues and 

 fluids a distinct and characteristic coloration. Notwithstanding the 

 evident physiological importance of these substances, and the striking 

 character of their optical properties, they have proved in many re- 

 spects more difficult of study than the other proximate principles ; 

 and with regard to several of them our knowledge is still very imper- 

 fect. In some instances this is partly due to the comparatively small 

 quantity in which they occur, in others to the extreme readiness with 

 which they are decomposed or altered in the process of separation. In 

 some cases it has been found difficult to decide whether a variation of 

 tint be due to the different proportions of one or more different color- 

 ing matters or to the varying degrees of concentration of a single one. 



The coloring matters are all nitrogenous compounds, but differ in 

 essential particulars from the albuminous substances. Those which 

 have been most fully examined are known to be crystallizable ; and it is 

 possible that all of them might be obtained in a crystalline form, could 

 they be completely separated without decomposition. The most abun- 

 dant of all, and that which possesses the most important physiological 

 properties, is the red coloring matter of the blood. It appears to be 

 analogous in many respects to the green matter of the leaves and leaf- 

 like organs in the vegetable world. Each of these two coloring matters 

 is the most abundant and widely diffused in its own kingdom, and is 

 distinguished by the identity of its characters in many different species 

 of animals and plants respectively ; and while the red coloring matter 

 of the blood, on the one hand, is the agent by which oxygen is absorbed 

 and distributed in the animal body, on the other, it is the green coloring 

 matter of plants by which carbonic acid and water are decomposed and 

 oxygen set free in the act of vegetation. It is believed by many ob- 

 servers that all the coloring matters of the animal body, at least in man 

 and the vertebrate animals, are derived by transformation from the 

 coloring matter of the blood ; and although we have no complete experi- 

 mental evidence that this is true in all cases, yet it is evident that these 

 substances have a close physiological relation with each other, perhaps 

 as distinct and real as that which exists between the various members 

 of the albuminous or saccharine groups. 



The organic coloring matters may be conveniently removed from 

 liquids containing them by the action of animal charcoal; that is, 

 (94) 



