126 MICROBES, FERMENTS, AND MOULDS. 



them. But this cannot always have been* the case, 

 and it has been supposed that the phenomenon of 

 coal formations, still so obscure and so variously 

 explained, was, at any rate, partially due to the 

 physiological labour of these microbes, and con- 

 sequently belongs to the class of fermentations. 



XIV. CHROMOGENIC MICROBES. 



In addition to the colourless microbes, such as are 

 most of those we have hitherto considered, there are 

 others remarkable for their vivid and varied colours, 

 which betray their existence to the least practised 

 eyes. Many of these microbes attack our alimentary 

 substances, and should therefore be known to the 

 manufacturer and hygienist, since their action on 

 the human system is often injurious. 



Many phenomena which have struck the imagina- 

 tion of ignorant and credulous people are merely due 

 to the presence of these coloured microbes. In 1819, 

 a peasant of Liguara, near Padua, was terrified by 

 the sight of blood-stains scattered over some polenta, 

 which had been made and shut up in a cupboard on 

 the previous evening. Next day similar patches 

 appeared on the bread, meat, and other articles of 

 food in the same cupboard. It was naturally regarded 

 as a miracle and warning from heaven, until the case 

 had been submitted to a Paduan naturalist, who easily 



