involved in the Consiniction of Artillery. 245 



which is found chiefly in elongated masses, the tendency is upon the whole to place 

 themselves parallel to the principal surfaces. It would seem, therefore, at first, 

 that the law of aggregation, apparently so opposite, must depend upon totally 

 different, conditions ; it is, however, essentially the same. In wrought, as in 

 cast-iron, the principal axes of the crystals, tend to assume the directions of least 

 pressure throughout the mass, while exposed to pressure and heat, in progress of 

 manufacture. 



195. Let us take the most strictly normal structure, the 3rd ; for example a 

 round bar of rivet-iron, half an inch in diameter. This has been formed by the 

 pressure of the grooved rollers in directions transverse to the axis of the cylin- 

 der, pressing it smaller and smaller, and still elongating it from a short thick 

 mass, whose original structure, if broken, may have been that of 1 or 2, the 

 metal being constantly at a temperature at which it is as soft as lead. Heat is 

 evolving the whole time, as in the case of cast-iron in cooling; but the pressures 

 produced within the mass are of a different character, and arise from a different 

 cause. In cast-iron they arose from the contractions of the mass in cooling : in 

 the wrought-iron bar (relatively small in two of its dimensions, and, therefore, 

 little affected at all, by contraction in cooling) the internal pressures ai"e pro- 

 duced by the rollers : but their pressures are all in directions perpendicular to 

 the length of the bar; or in our round bar in the directions of the radii of the 

 cylinder. The direction of least pressure is, therefore, coincident luith the length of 

 the bar, and this is the direction in which the j)rincipal axes of the crystals arrange 

 themselves. The same is the case with iron, drawn into wire, where the direc- 

 tions of maximum pressure being manifestly in the plane of the " draw-plate," 

 the aperture in which, presses powerfully round the periphery of the solid 

 passing through it, that of least pressui'e is, as before, parallel to the length of 

 the wire ; and so are the (fibres or) crystals arranged. 



196. Heat, as increasing malleability and ductility, i. e., intermobility of par- 

 ticles, facilitates the arrangement ; but as iron is a ductile substance even when 

 cold, so heat is not essential to the molecular change in the arrangement of its 

 particles ; just as in cast-iron we saw that molecular transpositions may continue 

 long after the mass has become solid. 



197. This is as strictly in analogy with the observable facts of crystallization 

 generally in other bodies (whether simple or compound, ductile or rigid, passing 



VOL. XSIU. 2 K 



