28 MARINE PRODUCTS OF COMMERCE 



particle enlarges to a small cube, its weight indenting the liquid surface. A trans- 

 parent, four-sided, fence-like rib of crystal, caused by the evaporation from four 

 sloping brine filaments, forms around the four top edges of the little cube and 

 results in a tiny, delicate, dish-hke structure of salt with the original cube at its 

 bottom. The cube enlarges, becomes heavier, and finally settles shghtly, although 

 still prevented from sinking by surface tension. 



A second fence-like crystalline rib forms in this newly depressed area and 

 surrounds the first rib, thus enlarging the four-sided crystal dish. The processes 

 of accumulating weight, settling slightly, and forming another addition to the 

 rim of the dish are repeated until finally a most beautiful hopper-shaped structure 

 appears on the surface. Taken from the liquid and dried on a piece of blotting 

 paper, the hopper crystal, which may be, if carefully made, half an inch or more 

 across and a quarter-inch deep, consists at the apex of a relatively large cubical 

 crystal, with four outwardly flaring, delicately ribbed, thin shell-like flake walls. 

 The corner "seams" are constructed of a number of smaller cubical pieces, ar- 

 ranged like the steps of an Egyptian pyramid. 



The slower the evaporation the greater will be the proportion of "flake" to 

 "cube" in any one hopper. When first formed, the hoppers are very thin and 

 fragile; but, if allowed to remain undisturbed on the brine surface, the apex cube 

 and the corner cubes, or steps, enlarge, and the fragile walls thicken until the 

 hopper dips, fills with brine, and slowly sinks to the bottom. 



Not all of the original salt crystals develop into perfectly formed individual 

 hoppers. The new-formed particles exhibit strong afiBnity for each other and at 

 every opportunity gravitate together, sometimes forming a thin, flat mat com- 

 posed of httle cubes and flakes. Several partly grown hoppers may unite at 

 their edges to form multiple hoppers. 



If evaporation is allowed to proceed still more slowly and the brine surface 

 is not agitated, as in well-protected solar ponds or coarse salt grainers, the hoppers 

 will grow much larger and thicker before sinking. After a large hopper sinks, 

 it is filled with small hoppers and particles that have early lost their floating 

 balance and gone to the bottom, and it eventually becomes a solid pyramid-shaped 

 mass with a flat base. Under extremely slow evaporation, as by solar heat, there 

 finally develops a bunch of beautifully formed, massive, block-like crystals vwth 

 sharp edges and corners; this is a typical formation of crude solar salt. 



If the evaporation is repeated with brine near the boiling point, the escaping 

 water vapor sets up a local circulation in the liquid, agitating the surface and 

 partly preventing hopper formation. Consequently, the new made salt particles 

 sink while still in the small hopper stage, as either incomplete hoppers, single 

 cubes, or groups or bunches of united cubes. If the brine is allowed to boil 

 violently, as in a vacuum evaporator, hopper formation is prevented, resulting in a 

 product containing no flakes and only small cubes. Therefore, we may draw the 

 following conclusions: (a) Violent boiling or stirring always results in fine cubical 

 particles of salt with no hoppers, (b) Near the boiling point evaporation with 

 gentle stirring or local brine circulation produces a mixture of larger cubical 

 particles and small, thin-walled hopper crystals, (c) Slow evaporation without 

 agitation produces hopper crystals with a large proportion of thicker-walled "flake" 

 grains, (d) Very slow solar evaporation produces solidified and filled hoppers and 

 block-like cubes, but almost no fine cubes or thin-walled hoppers. 



