424 
MR. F. OSMOND AND PROFESSOR ROBERTS-AUSTEN 
Fourth, the border of prisms become restricted and the polyhedra dominant; both 
prisms and polyhedra being of small size (K ; see photo. 5). 
Fifth, the distinction between the groups is very slight (A1; see photo. 4). This 
is only true for rather more than half of the section, the rest of it would be more 
fittingly placed in the second category. 
The question arises—are the differences which we have signalised caused solely 
by the difference in the composition of the alloys, or can they to some extent be 
attributed to variations in the experimental conditions under which the bars w r ere 
prepared, such variations, for instance, as those which occurred in the temperature of 
the metal at the moment of pouring? We cannot as yet tell. It is, however, 
certain that there is no relation between either the structure of the mass, the 
appearance of the fractures, the melting point of the alloyed elements, and the 
mechanical properties given in the table on page 418. 
Every iron metallurgist who sees our photographs will probably think, at first 
sight, that the alloy with potassium (photo. 5) will possess the best mechanical 
properties and that the structure indicated by photograph 1 is deplorable. 
The fact is that from a mechanical point of view the alloy of gold with potassium 
is the worst of all the series, and photograph 1 might equally well represent alloys 
which vary in tenacity from less than half a ton per square inch to 7 ’75 tons per 
square inch, and either are incapable of being stretched or may be elongated 32'6 per 
cent. 
There remains to be considered the groupings (polyhedral or prismatic), from the 
triple point of view of their external form, their internal structure, and their mutual 
relations in the neighbourhood of each other. 
II. Hitherto we have advisedly employed the vague term “ groups ” to designate 
the patches which are differentiated on the etched section by variations of colour and 
lustre. 
This is because we are uncertain as to the true nature of these groups, and it 
appeared useful to preserve a distinction which is ordinarily ignored between crystals 
which are defined by natural characteristic inclinations, and the pseudo crystals 
which have been arrested in a more or less haphazard way in consequence of the 
independent growth of neighbouring groups.* The latter groups have already been 
called “ cells ” or “ grains,” but the word grains appears in this case to be more 
suitable, in view of the fact that interposed foreign matter is usually absent. 
Microscopical examination, with a moderate enlargement of 100 to 300 diameters, 
enabled us to see the parts respectively played in our alloys by crystals and by grains. 
Pure gold when alloyed with 0’2 per cent, of bismuth, zirconium, rhodium, zinc, 
palladium or thallium, is entirely formed of grains. The gold alloyed with lithium or 
selenium only contains grains in the prismatic envelope, while in the interior the 
grains have a tendency to pass to crystallites, and the result is mixed jointing, with 
* The substance of the mass may, however, be crystalline. 
