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. 
7 
XIV. CHromocenic 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 
