IX] 



OF LIESEGANG'S RINGS 



42^ 



For instance, it would seem that, if the supply of -material to 

 the growing crystal be not forthcoming in sufficient quantity (as 

 may well happen in a colloid medium, for lack of convection- 

 currents), then growth will follow only the strongest lines of 

 crystallising force, and will be suppressed or partially suppressed 

 along other axes. The crystal will have a tendency to become 

 fihform, or "fibrous"; and the raphides of our plant-cells are 

 a case in point. Again, the long slender crystal so formed, pushing 

 its way into new material, may initiate a new centre of crystallisa- 

 tion: we get the phenomenon known as a "relay," along the 



Fig. 20H. Relay-crystals of common salt. (After Bowman.) 



principal lines of force, and sometimes along subordinate axes as 



well. This phenomenon is illustrated in the accompanying figure 



of crystallisation -in a colloid medium of common salt; and 



it may possibly be that we have here an explanation, or 



part of an explanation, of the 



compound siliceous spicules of 



the Hexactinellid sponges. 



Lastly, w^hen the crystallising 



force is nearly equalled by 



the resistance of the viscous 



medium, the crystal takes the 



line of least resistance, with ' 



very various results. One of 



these results would seem to be 



a gyratory course, giving to 



the crystal a curious wheel-like 



shape, as in Fig. 207 ; and other results are the feathery, fern-like 



207. Wheel-like crystals in a 

 colloid. (After Bowman.) 



