812 ON THE CRYSTALLIZATION OF CAST IRON.

a large hammer. This will generally be followed by afine display of crystallized iron, of every variety ofcolour; or, if the coloured crystals should be scarce,they can readily be made from the silvery coloured ones,by heating them in a vessel in a bright fire, till the sum-mits of the pyramids begin to change colour. The spe-cimen is then thrown out, and will be found, when cold,possessed of every shade of colour of which polishediron or steel is susceptible.

Cells of colourless crystals, resembling frosted silver,are sometimes observed in large castings, opposite tothe runner or feeder, which seem to differ from theother crystals merely in their formation having takenplace under circumstances where atmospheric air hasbeen totally excluded. The less carbon in the iron, thepurer the colour, and more perfect the form of the crys-tal ; and this observation may be extended to all crystalsof cast iron in general.

When cast iron is run from the blast-furnace, and be-comes covered with a quantity of the fluid slag, the sur-face of such iron is sometimes found to possess a linearchequered crystallization, in which are stems of the moreperfect pyramid, but of an imperfect form, like theletter ± with feathery lines across, and but little relievedfrom the plane of the metal. Iron cooled slowly in thiscase communicates the most beautiful prismatic coloursto the slags, of a deeper and more determined dye thanthe surface colours before alluded to.

If Swedish or Russian malleable iron, both of whichcontain more carbon than English iron, are melted in acrucible, they will cool with a slightly radiated convexupper surface, and a smooth under surface with theconvexity increased. If the same irons are melted underglass containing an alkaline mixture, both their upperand under surfaces become beautifully crystallized. Ifthe carbon predominates, as is the case in some of the