308 SCIENCE PROGRESS 



thus appear to be (i) gravity, (2) the adhesion of the sedimentary 

 layer to the upper surface of the vessel, (3) the cohesion in the 

 sedimentary layer, (4) the surface tension reaction between it 

 and the water, and (5) the friction of the water. 



" The network is therefore to be regarded as the resultant 

 of these forces, and appears to be explainable as follows : 



" When the film is turned upside down the force of gravity 

 immediately begins to act upon it, and the first effect is that 

 the adhesion to the glass plate or surface is weakened. It then 

 begins to sink. As soon as this takes place the cohesion of 

 the sedimentary layer asserts itself, and the particles come 

 closer together in some places, and separate in others, the 

 result being an irregular network. If the downward movement 

 due to the fall of gravity is slow, as when a denser solution 

 than water is used, the network is more regular in character, 

 and as the particles descend in the liquid slowly, they are 

 gradually attracted into more or less regular and equally spaced 

 groups. These aggregations, therefore, appear to be cohesion 

 phenomena, and the patterns produced are cases of cohesion 

 figures." 1 



We can obtain similar cohesion figures in a variety of ways 

 whenever we have a more or less homogeneous film placed 

 under such conditions that a force or forces acting in opposition 

 to the cohesion of the film or layer can be brought into play. 

 The network-like aggregations of Euglena are comparable with 

 those which are formed by precipitates or sediments and are 

 probably to a large extent formed in the same way. 



"A drop of water which contains a large number ol motile 

 Euglenae or other micro-organisms behaves just as a liquid 

 heavier than water, such as a drop of ink or of a solution of 

 cochineal. If it is placed on the surface of clean water it sinks 

 as a whole beneath the surface, expands into a vortex ring, 

 which gradually becomes larger but is slowly broken up by the 

 friction of the water and finally becomes diffused in it, just as 

 a drop of cochineal would do or a vortex ring produced by a 

 mixture containing a fine sediment. 



" If the Euglenae are much crowded but evenly distributed, 

 either in an open or closed shallow vessel, the first effect of 

 gravity is to cause them to sink. This brings them at once into 

 closer contact with one another and a dense layer is produced, 

 in which phenomena of cohesion are apparent in the formation 

 of a well-marked reticulation. 



" If the number of Euglenae in the water is only sufficient 

 just to give it a green colour, this downward movement does 



1 Wager, loc. a'/., p. 375. 



