SPECIFIC GRAVITY OF VERTICAL-POURED CASTINGS. 381
castings are tested a few inches from the bottom and a few inches from the top, the reason for finding the upper end the denser, as exhibited by the tests recorded, the author defines as being largely due to the law of metal expanding at the moment of solidification. Expansion tending to make the upper end of castings as dense as the lower may be better understood when it is stated that molten metal begins to solidify at the bottom of a mould and rises in height as the solidification continues. The effect of expansion at the moment of solidification, as castings '' freeze '' from the bottom upwards, has a crowding action, tending to make the molecules denser as solidification increases, thereby partly neutralizing the effect in the difference of the specific gravity naturally expected to exist while the metal is in a fluid state. The author has obtained the following Table 85 of analyses of the top and bottom piece of the vertical-poured parallel gate test bar from E. D. Estrada, M. E., of Pittsburgh, Pa.:
Carbon. Phosphorus. Manganese. Silicon. Sulphur.
Top piece ......... Bottom piece... 3-72 3.81 0.091 0085 0.31 0-33 1.32 1.32 0.046 0.047
These results show that practically there is little difference in any chemical constituent that might tend to equalize the specific gravity of the two ends of the vertical-poured parallel gate test bar, and that we are left to accept the author's theory of such results being due to the principles involved in the rate of cooling and by expansion at the moment of solidification. unnatural effect that cannot be charged to specific gravity proper.