26 
MESSRS. C. T. HEYCOCK AND F. H. NEVILLE ON 
7} -j- liquid = H. Etching with HCl leaves such an alloy at first with the t) and H 
both pure white and the mother-substance dark, but in time the rj darkens through 
oxidation, while the l)order of H remains of the purest white. There is thus no 
difficulty in distinguishing the two substances (fig. 92). 
'The H Matei'ial. —This substance appears to resist completely the action of even 
the strongest HCl, or of mixtures of the acid with ferric chloride. It also resists the 
slow oxidising action of the air much better than 17 , remaining a very pure white for 
days or months. We find it bordering the > 7 , as in the lower chills of Sn 29, in round 
or oval .spots, as in the prolonged mercury-boiled ingots of Sn 50, and in definite 
crystalline sliape in chills of Sn 90 (fig. 89). 
The Evidence from the Microscopic Structure of the Chilled Alloys. 
The AB Alloys. —As will be seen in the cooling curve of Sn 4, these alloys present 
but one halt in tlieir cooling curves, so that, as far as such evidence can be trusted, 
they appear to solidify in one continuous process ; we hope that the micro-j)hoto- 
graphs will be found to confirm this view. It is evident that if Sn 2 and Sn 4 both 
solidify in this manner, then the solid in these alloys cannot be a pure body, but 
must be a solid solution. 
Sn 1 . 1'85 per cent, hy weight of tin (Plate 1 , fig I). 
In unchilled and moderately slowly cooled ingots polishing, even without etching 
or ignition, brings out a j^attern of long .slender combs well seen with a power of 
10 diameters. This pattern is probably due to tlie more rapid oxidation of the first 
formed a, which was very rich in copper. Heat-oxidation develops the same combs, 
which do not fill more than three-quarters of the area. The lobes of a comb are not 
isolated, but form a continuous skeleton. In every case a higher magnification makes 
it evident that these combs are not sharply divided from the material round them, 
but fade softly into it. In fact, the margin of a comb does not mark an abrupt 
change in the process of crystallisation. This important point became very evident 
in a section etched electrolytically by being made the anode in a cell containing 
dilute sulphuric acid. This method of etching revealed the margins of the real 
skeleton crystals. They have sharp edges, and occupy at least 99 per cent, of the 
area. It is thus evident that the combs brought out by polishing or ignition, as seen 
in fig. 1 , are cores to the real skeletons of a, these latter being so closely packed that 
it is only here and there that we can detect their boundaries. Tlius, in spite of the 
fact that the cores are somewhat richer in copper than the outside la 3 mrs, we feel 
justified in stating that the alloy solidified in one continuous process. Moreover, the 
cavities between the real margins of the combs show that these combs, which so 
nearly fill the field, are primary, that is to say, were formed during the process of 
solidification, the small cavities being caused by the presence of gas or by the 
