92 SCIENCE PROGRESS 



however, only slowly ; in specimens which have been cooled 

 comparatively rapidly from the molten state, as under ordin- 

 ary casting conditions, a distinct structure is visible under the 

 microscope. A section of an ingot of brass of this composition 

 is shown in fig. 2. The irregularly polygonal boundaries of the 

 crystal grains are seen as before but the area within each grain, 

 instead of being entirely uniform, as in the ingot iron, is 

 marked with " dendritic " patterns which are evidently of the 

 nature of the crystal skeletons described previously. They are 

 visible in the brass, although invisible in the iron, because the 

 alloy freezes in a manner which is somewhat different from 

 the freezing of a pure metal. The first particles of solid which 

 crystallise from the molten alloy are relatively richer in copper 

 than the liquid and the subsequent accretions to the original 

 nuclei contain a diminishing proportion of copper. There is 

 thus a distinct difference of composition between the material ol 

 which the primary and secondary axes are composed and that 

 with which the gaps between the axes are filled up. If an 

 etching-agent be used which attacks the portions richest in zinc 

 most readily, the parts of each crystal grain which are in 

 contact with the boundary are most etched and appear dark, 

 whilst the central axes appear as light " cores." This cored 

 structure is characteristic of cast homogeneous materials, in- 

 cluding brass, gun-metal and many of the special engineering 

 alloys. Theory teaches us that equilibrium is only reached 

 when the composition of the mass is rendered uniform through- 

 out by diffusion of one of the constituents from places of high 

 to those of lower concentration. This diffusion, however, has 

 to take place in a solid the internal viscosity of which is very 

 great and the equalisation of composition is therefore a slow 

 process. Annealing the alloy at a sufficiently high tempera- 

 ture greatly facilitates diffusion and a specimen of the same 

 brass after thorough annealing exhibits a perfectly homogeneous 

 structure in which no cores are to be seen. Fig. 3 represents 

 the same specimen as fig. 2 after heating to redness during 

 several hours. The light and dark areas are of the same com- 

 position and differ only in orientation. 



Mechanical work produces a great distortion of the crystal 

 grains in metals and alloys of the above class and the outlines 

 of the broken and distorted grains may be barely distinguishable 

 in a thoroughly worked metal. Annealing brings about a 



