ISO Mr. Mallet on the Physical Conditions 



gation of crystals ; but by no means the most regular arrangement 

 of them, which depends chiefly vipon — 

 3°. The rate at which the mass of the casting has cooled, and the regu- 

 larity with which heat has been carried off by conduction from 

 its surfaces to those of the mould adjacent to them ; and hence it 

 is, that of all castings in iron, those called " chilled," that is to say, 

 those in which the fluid iron is cast into a nearly cold and very 

 thick mould of cast-iron, whose high conducting power rapidly 

 carries off the heat, present the most complete and perfect deve- 

 lopment of the crystalline structure perpendicular to the chilled 

 surfaces of the casting. In such the crystals are often found 

 penetrating an inch and half or more into the substance of the 

 metal, clear and well defined. 

 18. These prevailing directions of crystalline arrangement may be made 

 more clear to the eye by the Plate No. ii. 



Figs. 1 and 2 are sections of a round and a square bar of any of the crys- 

 talline solids we have spoken of, or of cast-iron, when the crystallization is well 

 developed (the circumstances affecting which we shall consider further on). 

 In the round bar the crystals all radiate from the centre ; in the square bar 

 they are arranged perpendicularly to the four sides, and hence have four lines 

 (in the diagonals of the square) in which the terminal planes of the crystals 

 abut or interlock, and about which the crystallization is always confused and 

 irregular. 



In Fig. 3 a flat plate is shown in section. The directions of the crystalline 

 axes follow the law of Fig. 2, with an extension in one direction. 



In Fig. 4 a section is shown of the hollow cylinder of lead alluded to 

 (page 8), in which, as in the case of Fig. 1, the arrangement of the crystals is 

 always towards the centre, or axis of the cylinder. This figure also applies to 

 every cast-iron hollow cylinder, whether water-pipe, gun, mortar, &c. &c. 



Fig. 5 represents a portion of the lower or closed end of the cylinder of 

 the hydraulic press as first made for the purpose of raising the tubes of the 

 Britannia Bridge, and which broke in the attempt ; the end of the cylinder 

 having broken out from the sides in the form of a flat frustrum of a cone, as in 

 Fig. 5 B, under the severe water pressure to which it was exposed ; that is to say, 



