220 Miss C. A. Raisin. On certain Structures formed 



ing as an axis, and shallower narrower furrows forming the- 

 branches. 



In materials which consist of very finely divided particles, the 

 variation in the size of the grains is too slight to produce any effect 

 and the result is a uniform pattern. Thus indigo generally forms a 

 typical example of a maze. If, however, the material is smooth and 

 oily, like gamboge, it is very slow in drying, and the general form 

 which it assumes is that of knobbed straight axes. 



In powders of medium grain the difference in the size of the 

 particles causes marked effects (e.</., chalk, prussian blue, sepia, &c.) 

 If the mud is dense, drying uniformly, a mazy network is usually 

 formed. A thinner mixture may produce knobbed straight axes. 

 An intermediate condition in the drying mass gives rise to feathery 

 or fan shapes, or to parallel generally curving bars. 



The mode of formation can be understood by tracing successive 

 stages, and by comparing different examples. When the water begins 

 to evaporate, the retreating edge of the film drags back the coarser 

 particles, leaving behind some of the finer material in a moist con- 

 dition. This afterwards dries, not however quite uniformly, so that 

 it is ranged in wave-like lines, between which clear spaces or uniform 

 thinner material may intervene. If the boundary is wide the lines 

 will be almost straight, if the margin is narrow they will curve with 

 smaller radius. The film of finer material, after retreating, some- 

 times advances again in a slightly different direction as was seen in 

 watching it, and two sets of curving lines are thus formed, which 

 partly cross one another. 



Meanwhile as the thick film retreats, its rim becomes concave 

 between certain points. Then the boundaries of adjacent concavities 

 approach, and in this way the intervening material is gradually 

 reduced to a streak. Inequalities, however, exist within the film, 

 and, where coarser grains occur, or a clot adheres closer to the glass, 

 these scrape away the finer material, thus clearing paths and leaving 

 intervening streaks, which are radial, often curved and transverse to- 

 wave-lines. The clearer furrows may terminate at a knot of coarser 

 grains, when the force no longer suffices to move these back, so that 

 they remain isolated or form the initial knob of a thick stem. This 

 may extend as a straight axis, or, if the film of air broadens as it 

 indents and pushes back the muddy fluid, the curving margin may 

 form the beginning of a maze or network, such as was described 

 above. 



Between the thick axes fine stuff is distributed on similar prin- 

 ciples to those which caused the forms near the edge. The two 

 dominant structures are the nearly parallel or concentric wave-lines 

 and the radial streaks. But in this case the finer material is 

 deposited within limited areas by which the forms are modified. If 



