Dr Brewster on a aingular Siructure in tlu Diamond. 99 
Had the diamond not been placed at the head of the mineral 
kingdom, from its unrivalled lustre, and high value as an orna- 
mental gem, it would have attained the same distinction from its 
great utility in the arts. Separated from all other gems by its 
remarkable refractive power, and from all mineral substances by 
its extreme hardness, its chemical composition, and its locality, 
in the crust of the earth, it has always been regarded as an ano- 
malous substance, which set even speculation at defiance. 
When Sir Isaac Newton compared the refractive power of se- 
veral bodies, he remarked that amber and the diamond had a 
refractive power three times greater, in respect of their densities, 
than several other substances, and he conjectured that the 
diamond was ‘‘ probably an unctuous substance coagulated.” 
• This relation between the inflammability of bodies and their ab- 
solute refractive power, I had an opportunity of confirming and 
extending, by ascertaining that Sulphur and Phosphorus ex- 
ceed even the diamond in absolute refractive power, and that th^ 
three simple inflammable bodies stood at the head of ^11 other 
solid and fluid substances in their absolute action upon light 
In this arrangement Amber stood next to Diamond^ and as 
both these substances had a similar locality, and had also carbon 
for their base, it became of some importance to discover that 
their general polarising structure was the same. The analogy, 
however, to which I wish to direct the attention of the Society, 
is founded on the existence of small portions of air within both 
substances^ the expansive force erf which has communicated a 
polarising structure to the parts in Immediate contact with the 
air. This structure is displayed by four sectors of polarised 
light encircling the globule of air, and can be produced artifi- 
cially either in glass or in gelatinous masses, by a compressing 
force propagated circularly from a point. It is obvious that 
such an effect cannot arise from any mode of crystallization, 
and if any proof of this were necessary, it might be sufficient 
to state, that I have never observed the slightest trace of it in 
more than 200 mineral substances which I have examined, nor 
in any of the artificial salts formed from aqueous solutions. It 
can therefore arise only from the expansive force exerted by the 
G 2 
Treatise on New Phil. Tnatruments, p, 266. 
