ALG/E. 1 1 3 



species, is the sudden change of their specific gravity, 

 which causes them to rise suddenly from the bottom 

 of the water, where they are developed in vast num- 

 bers, to the surface, and as suddenly sink down again. 

 He instances the river Leba, near the Prussian 

 frontier, and observes that " this Leba is a true moor 

 river ; its banks are quite flat, the bed is nothing but 

 moor and swamp, which gives way under one's feet. 

 Whenever the river is about two feet deep, the water 

 takes a brown colour, which prevents people froni 

 seeing to the bottom. On July, 1877, the river 

 appeared quite green, from a vast quantity of minute 

 spherical bodies which floated on its surface, and 

 even ordinary people were struck by it. The phe- 

 nomenon, which was first noticed towards noon, 

 lasted for about five hours, and had totally disap- 

 peared in the evening. The next morning there was 

 nothing to be seen, but at noon there was again a 

 large quantity, whilst there were very few towards 

 night. It was similar on the third day ; but since 

 then the minute spherical bodies have entirely dis- 

 appeared from the Leba." ^ There does not appear 

 to have been any further determination of these 

 spherical bodies, than that they received the tempo- 

 rary name of Rivularia fliiitans^ which, it is just 

 possible, was the same as the Rivularia hereafter 

 named. The phenomenon was much more transient 

 than in the case of the Shropshire meres. 



A report made upon what was called the "breaking 



• M. C. Cooke, Introduction to " British Fresh Water Algoc," p. 167. 

 (1890.) 



* Hed'i'igia (1878), vol. xvii. p. i. 



I 



