LV>8 PYOCYAXIX. 



and rhodophane (p. 263), and is slowly bleached by the action of 

 light. 



The pigments of the animal body which have been so far dealt 

 with admit of a certain amount of classification with reference 

 either to the secretions or organs in which they occur, to their 

 genetic relationships each with the other, or in some cases (lipo- 

 chromes) to their probable chemical similarities. But in addition 

 to these an extremely numerous mass of pigments has been at dif- 

 ferent times described under various names, as obtained from the 

 brightly -coloured parts of invertebrates and of vertebrates, such as 

 the feathers, &c. Our knowledge of them is quite incomplete and 

 limited in most cases to statements of their solubilities and the 

 absorption spectra which some of them yield. In most cases nothing 

 is known of their chemical nature or their relationships (if any) 

 to each other, and any description of them even if it were profit- 

 able, is impossible within any reasonable limits. 



For details and references to the literature of the several pigments 

 see Gamgee, Physiological Chemistry, Vol. i. 1880, p. 305, and parti- 

 cularly Krukenberg, Vergleichend-physiol. Studien, Heidelberg, 1881- 

 1888 and Vergleich. physiol. Vortrage, Bd. i. 1886, Nr. 3. 



In conclusion it must suffice to describe two pigments which do 

 not naturally fall under any of the above groups into which these 

 substances have been divided. 



Pyocyanin. 1 Pus, which ordinarily presents a more or less 

 bright yellow colour, is frequently greenish and sometimes blue. 

 The blue colour is due to a pigment (pyocyanin) which is ap- 

 parently formed in the pus by the action of specific organisms. 

 It is obtained either from pus or the bandages into which it has 

 been absorbed by extraction with dilute alcohol or with water to 

 which a trace of ammonia has been added. The alcoholic extract 

 is then evaporated to a small bulk and the residue extracted with 

 chloroform, or it may be extracted at once from the aqueous solu- 

 tion by shaking with chloroform. It may be obtained in a crys- 

 talline form by slow evaporation of the chloroformic solutions, the 

 crystals being readily soluble in water and alcohol, but only slightly 

 in ether. Acids change the blue colour to red, and alkalis restore 

 the original blue. None of the solutions show any distinct ab- 

 sorption bands. When kept the crystals turn greenish, due to a 

 decomposition which takes place most readily in alkaline solu- 

 tions exposed to the air and light, and results in the formation of 

 a yellow pigment, pyoxanthose. The latter is, unlike pyocyanin, 



1 Fordos, Compt. Rend. T. LI. (1860), p. 215; Ibid. LVI. (1863), p. 1128. Liicke, 

 Arch. f. Uin. Chirurg. Bd. in. (1863), S. 135. Girard, Deutsch. Zeit. f. Chirurg. 

 Bd. vii. (1876), S. 389. Fitz, Ber.d. d. chem. Gesell. Bd. xi. (1878), Sn. 54, 1893. 

 Kunz, Monatsh.f. Chem. Bd. ix. (1888), S. 361. 



