in the drying of a Fluid wit/i Particles in Suspension. 219 



like vesicles towards the surface of an igneous sheet or 

 dyke. Still more interesting are tapering pointed canals, some of 

 which radiate from the bubbles or from a central point. Others are 

 generally branched, often slightly curved, and have a central granu- 

 lar streak. Near the sealed edge, canals are numerous and vesicles 

 are large and few ; while beyond this part, the canals diminish in 

 number and finally almost entirely give place to numerous but small 

 vesicles. The intervening mud exhibits various contraction cracks 

 which originated in drying. 



Thus, the structures may be classified as : 



1st. Those of coarser threads, 



(1) where these form a maze ; 



(2) where they produce straight axes usually with terminal (or 

 initial) knobs ; 



2nd, the finer pattern, 



(3) consisting of concentric or wave-lines ; 



(4) forming feathery or fan-shaped tufts ; 



3rd (5) vesicular structures, including rounded vesicles, or taper- 

 ing canals ; 



4th (6) contraction structures, cracks. 



This classification is somewhat artificial and intended to indicate 

 the chief types, between which gradations may be found ; and by 

 combination and aggregation of different forms numerous varieties 

 arise, as may be seen in the few figures reproduced. 



3. Conditions and Causes of Formation, 



It is not always easy to describe concisely the exact conditions that 

 govern the formations since they include at least three variables. 

 First, the nature of the material, its specific gravity, adhesive 

 character, and the size of the grains ; second, the relative amount of 

 the material, i.e. the muddiness of the fluid mass ; third, the form of 

 the area occupied and the direction of retreat during subsequent 

 drying. As an example of the influence of the last-named condition : 

 in one case, when indigo was carried in by a current, the coarser 

 particles were deposited over a triangular or semicircular area, 

 while the finer material spread over the whole space.* Further, any 

 pressure or force acting upon the mass during drying may influence 

 the form assumed ; thus bubbles sometimes exercise a resistance on 

 the part surrounding. f 



Materials which consist of grains too large and heavy to be easily 

 moved may give rise to a rude attempt at a pattern. Thus, ordinary 

 fine sand exhibited a dendritic form, a deep channel or gutter extend- 



* The corner of the cover-glass was propped up with a few bristles. 



f Included bubbles sometimes are surrounded by concentric bands of the finer 

 material, as if the bubble favoured a uniform evaporation. 



