of Edinburgh, Session 1880-81. 
23 
substance alone. These crystals I believe to be true diamonds,, 
which are coloured black by amorphous carbon being disseminated 
through them ; indeed, a number of them are not entirely dark- 
coloured but transmit light, although not sufficiently to admit of 
their action on polarised light being given. To account for their 
being coloured in this way, it may be conceived to have occurred in 
the following manner, as far as the present experiments show : — 
The metal on being fused and kept at a temperature consider- 
ably above its melting-point, and thus very liquid, dissolves the 
amorphous carbon until it becomes a perfectly saturated solution. 
In this state it can take up no more carbon, but there being a large 
excess of that substance in the impalpable amorphous state present, 
it becomes disseminated throughout this liquid mass, and when the 
latter crystallises the amorphous carbon is naturally enclosed in the 
crystals both of carbon and silver, and thus colours the former. 
This is also the source from which the amorphous carbon comes 
that as before -mentioned occurs stuck together in little flocks, along 
with these crystals, on dissolving the metal, only that of course it 
was enclosed in the crystals of the metal. This is so perfectly 
evident that it is unnecessary to say more about it here. 
The question to be decided, then, is as to whether these dark 
crystals are really diamonds ? If we consider their method of pre- 
paration from pure sugar charcoal, in charcoal crucibles, with pure 
metal, we see that the only substances they can possibly be are 
carbon, silver, or platinum (when an alloy of silver and platinum is 
used). I have not been able to detect the presence of either of the 
latter substances in them, and consequently believe them to be pure 
carbon. Moreover, their crystalline form, the perfect octahedra 
witli curved edges, is the form of crystal peculiar to the diamond 
and to it alone, which makes one the more certain that these crystals 
are real diamonds though coloured. Again, the presence of graphite 
makes it more probable, and shows that the carbon must have been 
in such a state of molecular disaggregation as to allow it to pass 
from the amorphous form to the crystalline condition, and if so, 
why should it not take the diamond form as well as the graphitic 
one 1 which is in fact what has actually taken place. Although, as 
I have remarked above, the tendency is for the greater portion of 
this dissolved carbon to crystallise in the hexagonal or graphitic 
