Mr. A. Dick's Contributions to the Metallurgy of Copper. 421 



in the centre of which there is always a depression, owing to the 

 contraction of the metal during solidification. In this depres- 

 sion are generally seen a number of crystalline points. The 

 appearance of copper which has been melted under charcoal is 

 influenced by a number of circumstances. One of these is de- 

 serving of special notice, because it gives rise to one of the most 

 marked characters of overpoled copper, viz. the " rising in the 

 mould.'' 



AATien copper which has been melted under charcoal is cast 

 into an ingot mould under ordinary circumstances, the ingot, 

 when cooling, gives off a gas, sometimes causing projection 

 into the air of small globules of the metal, and it solidifies 

 with a very rough but tolerably flat surface. At other times no 

 projection of globules takes place, and the ingot cools with a 

 smooth surface; but in this case it is not flat; for just at the 

 moment of soUdification a quantity of still fluid metal is squeezed 

 from the central portion of the ingot towards the centre of the 

 upper surface, producing a ridge along it. '\^Tien such ingots 

 are fractui-ed, they present different appearances. In the case of 

 the former, which solidified with a rough surface, the fracture 

 shows numerous tubular caWties which have smooth and bright 

 metallic surfaces. The general direction of these cavities is from 

 the sides and bottom of the ingot towards the centre of the upper 

 surface — that part which solidified last. Many of them may be 

 traced from the sides to the top, where they end in little craters 

 formed bv the escaping gas at the moment the ingot was be- 

 coming solid. In addition to these larger cavities there are in- 

 numerable smaller ones, which cannot be discerned without the 

 aid of a lens, by means of which the whole substance of the 

 metal is seen to be quite vesicular. In the case of the latter 

 ingot, which solidified with a smooth surface and a ridge on its 

 centre, produced by the still liquid metal being squeezed from the 

 central portion of the ingot, the fracture is somewhat different. 

 There are no large cavities, but the whole substance of the metal 

 is seen to be uniformly vesicular, even by the naked eye. The 

 effect is manifestly due to the same cause, the difference arising 

 merely from the amount of gas evolved being greater in the one 

 case than in the other, or else from the quicker or slower cooling 

 of the metal. 



Between the two extremes there are of course all degrees. 

 Some ingots are minutely vesicular, and have a smooth flat sur- 

 face. Others have a few larger ca\-itics here and there among 

 the small ones, and a smooth surface with a ridge on the centre 

 of it. The specific gravity of a small ingot, made by melting 

 electrotype copper under charcoal and casting under ordinary 

 circumstances, which had this appearance, was found to be 7*851. 



