Overgrowths on Diamond. 
93 
et de grosseur ; mais il est presque toujours enveloppe d'une mince pellicule 
calcaire, qu'on enleve avec les autres impuretes qui pourraient ternir sa 
surface en le laissant pendant quelque temps dans de I'acide azotique 
bonillant." 
It is a curious statement to make, however, that diamonds fresh from 
the mine were almost always encased in a chalky envelope.* Certainly 
diamonds have always been put into acid — mostly aqua regia earlier, though 
hydrofluoric nowadays — before being sorted for shipment to London ; but^ 
as one understood, rather to remove impurities from cracks than especially 
to clean their surfaces, though of course it would do that too. And yet 
one hesitates to question the word of so careful a writer as Boutan. 
Possibly the explanation of the difficulty is either that such diamonds as he 
had the opportunity of seeing were thus coated, or that he mistook the main 
purpose for which the acid was generally used. True or not, the copyists 
lost a chance in not repeating the statement and so passing it on by their 
unanimity to the realm of historical fact. It may be mentioned here that it 
is well known at the De Beers pulsator that diamonds which have been 
lying for many years in the blue ground on the depositing floors are very 
intractable on the grease tables. Probably the long-continued weathering 
has set calcite free, either from the blue ground itself or from the limestone 
underlying the floors, to form an incrustation on the diamond. 
A clean diamond is not readily wetted by ordinary rain or river water. 
If it be immersed in such water it will come out dry saving where droplets 
can cling to irregularities upon its surface, and even these are easily 
shaken off. A triangular flake of diamond whose edges are 12 mm. and 
whose thickness is l"25mm. will float indefinitely upon a still-water surface. 
If the same flake be placed edge downwards in the water whose depth is 
about 6 mm., the lower edge will rest at the bottom of the water and the 
top corner will project slantwise through the surface, and remain so, pro 
vided that the surface is kept fairly quiet. This is the largest thin flake 
there has been the opportunity to try, though doubtless much larger ones 
would behave in the same way. Small diamonds of good geometrical form 
up to at least 0"1 carat each will float on water, even when it is gently 
agitated, like corks, partly because of the surface tension of the water, but 
chiefly because they are not readily wetted, whereas most common minerals 
of the same size will submerge at once. 
A number of simple experiments have been made with the object of 
ascertaining whether the addition of freely soluble substances to water 
would modify the surface tension between diamond and water. Saturated 
solutions of the following compounds showed no appreciable difference : 
* Cf. Gr. F. Williams, p. 208: "All the crystals in the blue ground were encased 
in a smooth bed of the same material which did not adhere to the diamonds, so that 
their lustre, when extracted, was quite bright or glassy." 
