226 Mr. Mallet on the Physical Conditions 



of all these, consists ia stirring up the mass of melted metal with a dry, hard- 

 wood stick. What occurs seems to be the removal of a portion of oxygen, 

 either directly combined, or in the state of a combined suboxide, from the mass, 

 by the afGnityof the hydro-carbons produced by the burning of the wood beneath 

 the molten mass. When this process is carried too far, the copper, in place of 

 remaining at " tough pitch," at which, when cold, it would be capable of being 

 forged, rolled, wire-drawn, &c., "goes back," and becomes brittle and short as 

 " tile copper" again ; and on examination it is found now, that the oxygen is gone, 

 but that it is replaced by a small proportion of carbon, which the metal has 

 absorbed. Oxygen is, therefore, present in the metal, but a slight play of 

 affinities is sufficient to cause its removal from the liquid metal, from which some, 

 at least, is probably evolved on its consolidation otherwise. But it is a fact well 

 known to gun-founders and bell-founders that the oftener the alloys of copper 

 and tin are melted, the more difficult it is to produce solid castings with them, 

 and that very frequently castings made with such metal, though presenting no 

 large or obvious cavities or defects, are yet found, upon examination with the 

 microscope, to be filled, almost with perfect uniformity, throughout the whole 

 mass, with innumerable minute vesicles, all of an almost perfectly spherical 

 form. In the case of large bells this is not uncommon ; and the author, in his 

 owni practice, once examined a bell of large weight, cast chiefly from old bell- 

 metal, so filled with vesicles of this sort that the want of homogeneity, interfered 

 with the sonoricity of the metal, and the bell gave scarcely any clear sound when 

 struck, and was, moreover, unusually brittle and incoherent. 



Now, it is an ascertamed fact that this difficulty of making sound castings 

 from old and often re-melted alloys of copper with tin (or with zinc) arises from 

 the oxidation of the tin and the copper, which the experiments of Dussausoy 

 showed, took place in such proportions that in the case of gun-metal, for one 

 part by weight of tin oxidated, from three to four of copper were so. A part 

 of this oxygen absorbed or combined appears, then, to be given up again by one 

 or by both metals at the moment of consolidation, and its evolution to be the 

 cause of the perfectly uniform dissemination of the minute air-vesicles alluded 

 to, which are seldom known to occur in such abundance in new or not frequently 

 fused metals, though more or less, they may almost always be found by micro- 

 scopic examination, of all gun and bell-metals. 



But again, where copper, or its alloys, have absorbed an excess of carbon, 



