1874.] Expansion of various Substances on Solidification. 367 



panded was calculated from the weight of its contents when the vessel 

 and water were at 60 F. 



We have now, after applying some small corrections, the elements 

 necessary for determining the specific gravity of the cast iron w r hich filled 

 the vessel when in the molten state, having the absolute weights of equal 

 volumes of distilled water at 60 and of molten iron. The mean spe- 

 cific gravity of the cast iron which filled the vessel was then determined 

 by the usual methods. The final result is that, whereas the specific 

 gravity of the cast iron at 60 F. was 7'170, it was only 6'650 when in 

 the molten condition ; cast iron, therefore, is less dense in the molten 

 than in the solid state. Nor does it expand in volume at the instant of 

 consolidation, as was conclusively proved by another experiment. Two 

 similar 10-inch spherical shells, 1-5 inch in thickness, were heated to 

 nearly the same high temperature in an oven, one being permitted to 

 cool empty as a measure of any permanent dilatation which both might 

 sustain by mere heating and cooling again, a fact well known to occur. 

 The other shell, when at a bright red heat, was filled with molten 

 cast iron and permitted to cool, its dimensions being taken by accurate 

 instruments at intervals of 30 minutes, until it had returned to the 

 temperature of the atmosphere (53 F.), when, after applying various 

 corrections, rendered necessary by the somewhat complicated conditions 

 of a spherical mass of cast iron losing heat from its exterior, it was 

 found that the dimensions of the shell, whose interior surface was in 

 perfect contact with that of the solid ball which filled it, were, within 

 the limit of experimental error, those of the empty shell when that also 

 was cold (53 F.), the proof being conclusive that no expansion in 

 volume of the contents of the shell had taken place. The central portion 

 was much less dense than the exterior, the opposite of what must have 

 occurred had expansion in volume on cooling taken place. 



It is a fact, notwithstanding what precedes, and is well known to iron- 

 founders, that certain pieces of cold cast iron do float on molten cast 

 iron of the same quality, though they cannot do so through their 

 buoyancy. As various sorts of cast iron vary in specific gravity at 

 60 F., from nearly 7' 700 down to 6-300, and vary also in dilatability, 

 some cast irons may thus float or sink in molten cast iron of different 

 qualities from themselves through buoyancy or negative buoyancy alone ; 

 but where the cold cast iron floats upon molten cast iron of less specific 

 gravity than itself, the author shows that some other force, the nature 

 of which yet remains to be investigated, keeps it floating ; this the author 

 has provisionally called the repellent force, and has shown that its 

 amount is, cceteris paribus, dependent upon the relation that subsists 

 between the volume and " effective " surface of the floating piece. By 

 " effective " surface is meant all such part of the immersed solid as is in 

 a horizontal plane or can be reduced to one. The repellent force has 

 also relations to the difference in temperature between the solid and the 

 molten metal on which it floats. 



