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422                       METALLURGY   OF   CAST   IRON.
in thickness, it would be il white " and hard as flint. In the former  case,  also, it would  show much less contraction   than   in   the   latter.    The   facts   go   to show that the length of time occupied in cooling a casting  or that molten metal  has  solidified, may often be^inore effective in causing different degrees of contraction and  hardness of -iron in a casting  from   ordinary used foundry iron, than any varying percentages of sulphur,'Silicon, etc., which exist in ordinary foundry iron.    Any  one giving  due   consideration to the points here raised   will be led to  concede   the   impracticability of formulating set rules for the contraction of castings, to be published as a universal guide to desired results in the dimensions of castings; but by a study of the phenomena here referred to, we will be in a fair position  to  determine  what  allowance should be made  for contraction, etc., when we are on the ground of action.    It is to be understood that reference is not made to the difference which may exist in the size of like castings from soft and hard iron, or variations due to the hardness of ramming and head pressure of molten metal on moulds, etc.   We are mainly dealing with the elements involved in the question of  contraction,   as   affected by  rapidity  o"f  cooling, stretching of iron, and variations in the thickness of metal, etc., in castings.
Stretching is possible and due to influences exerted by conditions in casting, cooling, and forms of patterns, which overcome or retard free contraction. It can make castings larger than the patterns from which they were made, and it also makes it possible to obtain acceptable castings which could not be secured were it not for the fact that iron can be stretched.                                                        J  I