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425                      METALLURGY   OF   CAST   IKON.                                                  j
tween 15 and 30 minutes after the pouring that the               !
restrained bar held about even pace with the free bar.               |
From this point on, the restrained bar keeps gaining               j
on  the  free  bar,  until  the  end, when the free bar             ^
stands about one and one-half degrees ahead of the               f
restrained or weighted bar's pointer, thus showing we can restrict contraction by power and that the period               \
of the greatest stretching of cast iron, cooling from a solidified state to the temper coldness of the atmos-               ;
phere, wherever there is any restraint upon its contraction,   is  that  ranging from   1,600  degrees   F. to               ; i,200 degrees F., or in color from a light to a dark cherry.
One reason for describing the above tests in the manner detailed is owing to the fact of a low silicon mixture being used with but two 5o-pound weights suspended to retard the contraction. Many other experiments were made, as will be shown further on.
In closely watching the movements of the pointers of the restrained and free bars as they contract, a wavering, quick, forward (and often backward) mo-tion, sometimes as far as one-half degree, will be plainly noticed in the restrained bar, while the free bar has a constant steady forward movement. The quick, wavering motion is occasioned by the resistance to free contraction, which the weights offer to the bar, and occurs when the contraction occasionally has sufficient power to overcome. the influence of the weights to stretch out the cooling iron. The fact that cast iron can be stretched is also often exemplified in heavy foundry work in the cooling of castings, examples of which in every-day practice the writer will cite further on.cibly illustrates the great reason why the founder has to fear sulphur in fuel, high-sulphur iron, and to            j