84- Mr. D. Mushet on the Crijsiallizulion of Iron. 



The 2(1 stage of crystiillization is marked by a separation of 

 a certain number of these lines into quadrangular prisms, 

 sometimes five, and in other instances seven lines, forming the 

 stamina of tbe forth-coming crystals. As the process of 

 shrinkage is carried forward, the quadrangular prisms sepa- 

 rate longitudinally, and a series of points begin to appear, 

 which are the summits of the more perfect forms. Both series 

 of lines become nearly of the same magnitude, are much 

 more accurately defined ; and such seems the effect of the per- 

 pendicular subsidence, that the form of the crystal is also af- 

 fected by the gravitation of the mass. The crystal may now 

 be compared to a spear head, with serrated edges, or like 

 some varieties of the Fern. In indigo-coloured crystals, the 

 stem is frequently of a splendid silvery colour, and at other 

 times gray, gold, or purple. 



It may be proper here to observe, that although all lines in 

 the crystallization of cast-iron, when the crystal is in its most 

 finished form, appear perfectly smooth ; yet, when examined 

 by means of a glass, present a regular succession of knobs, 

 whose convex surfaces, assuming a different colour from the 

 inferior planes, add much to the variety and beauty of the 

 structure. 



The last and most perfect stage of the ciy stall ization of 

 cast-iron takes place under the following circumstances: Iron 

 exceedingly divided by fusion ; a large mass with considerable 

 perpendicular height to admit of slow and insensible subsi- 

 dence ; a complete exclusion of atmospheric air ; such circum- 

 stances being present, large and deep cavities are formed, and 

 time is given for the crystal to form and disengage itself from 

 the more solid parts of the iron, so as to form groups or 

 masses of crystallization. Each individual crystal has now 

 -become double, a new series of ramifications is found crossing 

 the original stem at right angles. Though this new formation 

 does not always measure so nmch across as the other ; yet, 

 when the form is complete, the crystal may be viewed on four 

 different sides, exactly similar to each other, forming a qua- 

 dranoular pyramid, a section of which horizontally would be a 

 cross. The serrated edge is now confined to the summit of 

 the crystal, which commonly tei-minates in the figure of a 

 spear point ; but the inferior portions present slender yet di- 

 stinct and well-formed filaments, proceeding from the stem to 

 the extremity. 



These pyramids are of various proportions, some only equal 

 to their base in height ; while others are equal to twice the 

 base; some of them incline rapidly, and form sharp summits ; 

 whilst others rise more perpendicularly ; frequently they are 



found 



