116 BACTERIOLOGY. 



After the lapse of a few days, the brightest pigment 

 fades, and may even disappear. This alteration may be due 

 to the action of the air, although the bacterial products, 

 especially the enzymes, may assist in the change. 



Although the bacterial pigments have been the subject 

 of numerous investigations, their exact chemical nature is 

 still unsettled. In some instances, the pigment would seem 

 to be a fatty compound {lipochrome); in another instance, it 

 would appear to be a basic substance, allied to the ptomains 

 (pyocyanin) ; a.nd again, in other cases, it appears to be a 

 protein compound. 



The various pigments may be divided into two large 

 groups according to their solubility in water. The fluores- 

 cing bacteria, and the bacillus of blue milk afford the best 

 examples of soluble pigments. These pigments form on the 

 surface of the medium, where there is an abundance of 

 oxygen, and gradually diffuse downward into the medium. 

 In the other group the pigment is insoluble in water, and 

 soluble in alcohol. The Bacillus prodigiosus, violaceus, etc. , 

 are good examples of this kind. In these instances the 

 pigment exists as granules on the outside of the cells. In 

 only two organisms does the pigment seem to be insoluble 

 in water and in alcohol. 



It is possible for various articles of food such as meat, 

 milk, cheese, bread, etc., to develop growths of pigment 

 producing bacteria. The B. prodigiosus and allied forms 

 have been frequently met with in such "food epidemics " 

 'The miracle of the "bleeding host," as well as "bloody 

 milk," imputed in a former age to witch-craft, find an easy 

 ■explanation in the sudden development of chromogenic 

 bacteria. 



Phosphorescence. 



The extremely interesting, and rather mystical pheno- 

 menon of phosphorescence is common to the sea-water in 

 all parts of the globe. Salt-water fish and oysters, when 



