214 MESSRS. C. T. HEYCOCK AND F. H. NEVILLE 



SECTION III. 

 The Microscopic Study of the Alloys. 



Sections, usually transverse, were cut from the rods of alloy extracted for analysis, 

 and from other alloys, some of which had been slowly cooled. These sections were 

 polished on graded emery paper and when necessary finished with rouge. They 

 were then examined under the microscope both before and after etching. We found 

 bromine water or aqua regia the only satisfactory etching reagents, these being about 

 equally effective in developing detail. 



Most of the reproductions in Plates 4 and 5 are from photomicrographs taken with 

 an arc light, but in two cases, in which the photograph did not satisfactorily 

 reproduce the detail of the object, we give drawings by Mr. E. WILSON of 

 Cambridge, who has had a large experience in this kind of work. Except in the 

 case of some low power photographs, for which oblique light was used, the illumina- 

 tion of the alloy surface was normal, a Beck's axial illuminator being used. The 

 photographs are arranged in order according to the number of atoms of aluminium 

 in the alloy, so that they can be readily referred to while reading the text ; but 

 reproductions are not given of all the alloys described. 



However perfect photomicrographs may be, they rarely give as much information 

 as a direct examination with the microscope. The descriptions which follow are 

 therefore based on what was visible under the microscope rather than on the photo- 

 graphs, although we hope that the latter will confirm our statements. 



The different ingredients of the alloys have, in some cases, well marked colours, 

 but with different illuminants the tint of the same patch of alloy varies a good 

 deal. It was therefore desirable to select a standard method of illumination for 

 the eye examination. We employed a Welsbach gas-burner, an image of the flame 

 being thrown a little out of focus on to the surface of the alloy. The colours of the 

 various substances found in an alloy were then fairly constant, and could be employed 

 as an aid in identifying them in different sections. 



As a rule Zeiss' apochromatic lenses were employed, the powers ranging from 

 50 diameters to over 1000. 



It will be seen that the microscopic examination confirms in every respect the 

 information given by the freezing point-curve. The microscope reveals seven sub- 

 stances in the series of alloys, although they are never all found in the same alloy. 

 They are gold and aluminium, pure at A and J, the ends of the curve, and four 

 bodies which are nearly or quite pure at the points B, D, E, and H respectively. 

 The seventh substance is present in all alloys between E and H, but never in a pure 

 state. We shall sometimes refer to these as the B, D, E, H, and X bodies respec- 

 tively. 



Nearly all the alloys, when polished long enough on dry emery and rouge, acquire 



