PRISMATIC STRUCTURE IN IGNEOUS ROCKS 221 



friction only when they divide the liquid into hexagonal cells, as 

 can be shown by the same line of argument as is used to prove that 

 a uniform shell, under tension due to its own contraction, breaks 

 with minimum energy expenditure when it divides into hexagons. 

 Fig. 2 is a cross-section of one of these hexagonal cells, showing 

 how the currents rise in the middle of each prism and flow down at 



Fig. 2.— Cross-section of a hexagonal cell, showing how the currents rise in the 

 middle of each prism and flow down at the boundaries. 



the boundaries. The contour of the surface of the liquid is exagger- 

 ated in the figure, but the relief is quite sufficient to permit the 

 structure of the circulating liquid to be observed by various optical 

 methods. Fig. 3 shows three examples of these structures in a 

 melted wax, taken under different conditions of temperature and 

 thickness and before the final steady state of circulation had been 



attained. 



A state of subdivision into irregular cells of from four to seven 

 sides is attained in a few minutes, even in a viscous oil. These 

 cells then join and subdivide repeatedly until finally, if the condi- 

 tions are constant, a perfect system of hexagonal cells is produced. 

 Even when the liquid is originally in motion, convection cells form 

 which show little or no trace of the original direction of movement 

 of the liquid as a whole. 



