and Origin of the Diamond. 249 



Fi<r. 2. 



A 



Fig. 1. represents the. diamond considerably magnified. 

 At a and b are seen two minute cavities, which appear per- 

 fectly black, as if they were filled with opake matter. This 

 blackness, however, arises from the high refraction which 

 takes place at the concave surfaces of the cavity, as may be 

 proved by the application of a microscope, which exhibits a 

 minute pencil of light transmitted through them. Fig. 2. 

 shows the four luminous sectors around each cavity, as ex- 

 hibited by the agency of polarized light. When a plate of 

 sulphate of lime which polarizes a blue tint of the second 

 order of colours in Newton's scale is placed across these sec- 

 tors, so as to have its axis coincident with the radii of two of 

 the luminous sectors opposite to each other, and perpendicu- 

 lar to the radii of the other two sectors, its blue tint of the 

 second order is depressed, by that which is polarized by the 

 sectors, to a red of the first order in the sectors whose radii 

 are coincident with the axis of the sulphate of lime, and raised 

 to a whitish yellow of the second order in the other two sec- 

 tors. Hence, it follows that the character of the polarization 

 in the sectors is negative, like that of calcareous spar, and that 

 it has been produced by a compressing force acting outwards 

 from the cavities. 



I have, in my former paper, supposed that the compressing 

 force was the expansive power of air included in the cavity ; 

 but this, of course, is a conjecture, though it seems quite cer- 

 tain that it must have been a gaseous body. That it was not 

 a fluid is obvious, from there being no fluid in the cavities. 

 This was certainly the case in the cavities in amber and glass; 

 but it is possible that a fluid of very low refractive power may 

 exist in the diamond cavities without my being able to see it, 

 on account of the high refractive power of the gem. If this 

 should be the case, however, it will not be difficult to observe 

 it in larger cavities, if they should ever be discovered. 

 Third Series. Vol. 7. No. 40. Oct. 1835. 2 K 



