ON THE CRYSTALLIZATION OF CAST IRON. 809

This stage of the operation takes place when a smallchasm is formed by shrinkage of a proportional quantityof fluid iron. Its beauty and regularity will greatly de-pend upon the process being effected in the absence ofexternal air; wherever air has reached the surface ofthe iron, the sharpness of the lines, and a still moreminute and silvery ground work crystallization, are de-stroyed by oxydation, and the beauty of the specimenvery much impaired.

The 2nd stage of crystallization is marked by a sepa-ration of a certain number of these lines into quadran-gular prisms, sometimes five, and, in other instances,seven lines, forming the stamina of the forthcomingcrystals. As the process of shrinkage is carried forward,the quadrangular prisms separate longitudinally, and aseries of points begin to appear, which are the summitsof the more perfect forms. Both series of lines becomenearly of the same magnitude, are much more accuratelydefined ; and such seems the effect of the perpendicularsubsidence, that the form of the crystal is also affectedby the gravitation of the mass. The crystal may nowbe compared to a spear-head, with serrated edges, orlike some varieties of the fern. In indigo coloured crys-tals, the stem is frequently of a splendid silvery colour,and at other times grey, gold, or purple.

It may be proper here to observe, that although alllines in the crystallization of cast iron, when the crystalis in its most finished form, appear perfectly smooth;yet, when examined by means of a glass, present a regu-lar succession of knobs, whose convex surfaces, assum-ing a different colour from the inferior planes, add muchto the variety and beauty of the structure.

The last and most perfect stage of the crystallizationof cast iron takes place under the following circum-stances :—Iron exceedingly divided by fusion ; a largemass with considerable perpendicular height to admit of