148 Mr. Mallet on the Physical Conditions 



1 1 . But conversely, the same effect is found produced by the application of 

 lieat (far below that of fusion) to the surfaces of solids, which are capable of 

 solidification in either of the two states, homogeneous (amorphous) or crys- 

 talline, and have solidified in the former. Of such bodies many are known ; 

 for example, — many of the metals, glass, carbon in certain states, chalk when 

 crystallizing into marble under pressure and ignition, arsenious acid, realgar, 

 protoxide (litharge) and iodide of lead, ice; and amongst even organic com- 

 pounds, sugar, paraffine, &c. 



12. If a cylinder of lead, of some four or five inches long, and about the same 

 in diameter, be cast around a cylindrical bar of iron, of about 1^ inches diameter 

 and some 2 or 3 feet long, the lead, on becoming cold, and rapidly consolidated 

 by the contact of the cold iron bar interiorly, will have a perfectly homoge- 

 neous structure ; it may be cut into, beaten out, &c., without presenting any 

 trace of crystallization. 



If, however, one of the projecting extremities of the central iron bar be 

 now placed in a furnace and heated red hot, and time be given until the heat 

 conducted along the bar, and from it passed into the interior parts of the 

 lead cylinder, and thence transmitted outward, radially through it in all di- 

 rections, shall have raised the temperature of the lead itself to within a few 

 degrees of its melting point — say to about 550° Fahr. — and the lead be now 

 struck sharply with a hammer, the whole mass will be found to have assumed 

 internally a crystalline structure, all the principal axes of the long thin crys- 

 tals radiating regularly outwards from the axis of the cylinder to its surface ; 

 and by a few blows of the hammer, the whole mass will separate and fall to 

 pieces as a metallic dust — so complete are the planes of separation of the 

 crystals. (See Plate n. Fig. 4.) 



13. A piece of cylindrical brass wire, tough, longitudinally fibrous, and pre- 

 senting no trace of crystallization, may in the same way be caused to become 

 almost instantly brittle and crystalline, if passed endways into the centre of a 

 red-hot iron tube of small diameter (such as a gun-barrel), held vertically; the 

 crystals all radiating from the axes of the cylinders. 



14. If a flat thick plate of rolled or malleable zinc, which is nearly homo- 

 geneous in structure, or, if not so, presents fibres and lamina in the plane of the 

 plate, be laid down flat upon a cast-iron plate, heated to within a few degrees 



