2i6 Mr. Mallet on the Physical Conditions 



through an intermediate plastic state, or crystallizing per saltum), which crys- 

 tallize in bacillary or fasciculated crystals, as were the analogies we found in 

 the case of cast-iron. Thus, for example, arragonite, tourmaline, gypsum, 

 actinolite, manganese-alum (from Cape Coast Castle), amianthus, &c., &c., are 

 all found frequently in embedded, more or less rounded fasciculi, of long, parallel, 

 fibrous crystals. When these are examined carefully with a lens, the external 

 crystals are always found more or less deformed by the pressure of the external 

 embedding matrix, to which they are moulded, though not formed by infiltration 

 and gradual filling of a mould. In every such case there are accompanying 

 evidences of great pressure in directions perpendicular to the longest dimension 

 of the bacillary mass. Thus nearly cylindric pencils of arragonite are found 

 so formed in the intensely compressed chalk, overflowed by huge incumbent 

 caps of basalt, in the north of Ireland. Similar pencils, though not cylindric, 

 of tourmaline, are found in granite; — manganese-alum, and fibrous gypsum, in 

 enormously deep beds of clays, which, when soft and plastic, transmitted the 

 pressure of their own mass of hundreds of feet in depth, with the fidelity almost 

 of a fluid; — amianthus in serpentines, whose configurations prove the former 

 play of enormous pressures, through plastic masses since become solid and rigid ; 

 and the instances might be greatly multiplied (Note E). 



Two of the examples given, arragonite and gypsum, present the remarkable 

 identity that they are found both in the arrangement of the crystals of cast-iron, 

 with their principal axes perpendicular to the bounding planes, and in that 

 parallel to them, as now described, for wrought-iron; in each case the arrange- 

 ment having followed the lines of least pressure, however produced, provided 

 it were coincident in time with such other conditions, whether of ductility, 

 plasticity, fusion, or liquidity by solution, as admitted of molecular transfer and 

 re-arrangement. 



198. Returning now to the wrought-iron rolled bar ; while its diameter con- 

 tinues small or moderate, although in the progress of its cooling internal strains 

 and variable pressures are induced by contraction, still, as almost any apprecia- 

 ble contraction is confined to the one direction, that of the bar's length, so these 

 new internal pressures are inoperative in producing any distinct changes in the 

 disposition given to the (fibre or) crystals in rolling. Not so, however, if the 

 bar, in place of being of small diameter, be very thick in proportion to its length. 



