CHEOMOGENIC BACTERIA 349 



was asked, could it be but blood, which could but mean 

 some terrible portent of great calamity ? It is needless to 

 say that great capital was made out of this ' miracle ' by 

 the Church in the Middle Ages. It was a miracle which 

 priest and layman could believe in with perfect honesty- 

 one of which, owing to the want of apparent cause, the 

 supernatural may have seemed the natural explanation. 

 ' Many of the bacterial colouring matters strongly re- 

 semble the aniline dyes in their behaviour to acids and 

 alkalies, and in their appearances on media. Some 

 cultures, after keeping, take on the peculiar metalUc lustre 

 so characteristic of the aniline dyes. Many of these 

 bacterial pigments are soluble in water, while others are 

 insoluble in water, but soluble in alcohol, ether, chloro- 

 form, etc. 



The colouring matter of the B. prodigiosus can be ex- 

 tracted with ether: that of the B. pyocyaneus is soluble 

 in chloroform. On extracting the growth from a number 

 of agar cultures of the B. pyocyaneus with chloroform, 

 and filtering, a deep-blue solution is obtained, which on 

 slow evaporation in the dark yields a crystalline residue of 

 pyocyanine. 



The cultural characters of a number of chromogenic 

 bacteria will be found in the last chapter. 



Phosphorescent Bacteria. — Many bacteria give rise to 

 phosphorescence as a result of their vital activity. It is 

 to these organisms that the beautiful phosphorescent phe- 

 nomena sometimes seeij in the sea, especially in the 

 tropics, are due. They are also seen not infrequently in 

 marshy places and on decaying wood ; the luminescence 

 occasionally exhibited by fish is also well known. The 

 light given off from the gelatine cultures of some of 

 these bacteria is suflScient to enable one to ascertain the 

 time by a watch in a perfectly dark room, and even photo- 



