224 Mr. Mallet on the Physical Conditions 



so loosely in combination, that very slight forces are sufBcient to induce its 

 segregation, into two or more different alloys, which, on cooling, are found to 

 occupy different portions of the mass, and possibly even to separate a portion 

 of one or other of the constituent metals. 



162. Thus, in a gun cast vertically, and permitted to cool slowly, as in the 

 ordinary practice of " loam casting," the external portions, which cool first, have 

 a determinate constitution, different from that assigned by the proportions of the 

 metals, as fixed for fusion. The interior of the gun, which cools last, has another 

 constitution different from either, and always richer in tin. But when the whole 

 gun has become solid, and portions are examined, from the extreme lowest, 

 middle, and highest parts, of the previously fluid column of metal, it is found 

 that these again differ from each other, and that this difference varies, in the 

 vertical for the exterior or crust alloy, which has cooled first, and for the 

 interior column of alloy that has cooled last ; so that, in fact, of any gun, no two 

 adjacent portions have strictly the same chemical constitution, — the maximum 

 of copper being foimd in the exterior and breech of the gun, or lowest part of 

 the column of metal; and the maximum of tin, in the interior and highest part 

 of the metallic column. The utmost discordance prevails in authors as to the 

 position in the gun in which the maximum of tin is found ; their conclusions 

 being almost always based upon isolated facts, and taking no account of the 

 differences that must arise from variations in the primary alloy, and in the 

 details and circumstances of its moulding, casting, and cooling, difference of 

 mass, &c. 



The account given must be viewed as the normal case, in which the homo- 

 geneous alloy is subjected to no forces in consolidating but those of its own 

 affinities, of gravitation and of cooling by radiation in a perfectly equable 

 manner. 



163. The segregation from the exterior to the interior is due to a play of 

 chemical affinity, separating the whole mass into two or more definite atomic 

 alloys, that of the exterior possessing less fusibility, and more copper, than that 

 in the interior, which remains fluid longer. 



The constitution of the latter was found by Dussausoy, to approach in 

 almost every case, an average constitution of (8 Cu + Sn). 



