and Origin of the Diamond. 459 



each cavity, as exhibited 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 sectors, so as to have its axis coincident 

 with the radii of two of the luminous sectors opposite to each other, and per- 

 pendicular 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 sul- 

 phate of lime, and raised to a whitish yellow of the second order in the other 

 two sectors. 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 con- 

 jecture, though it seems quite certain 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 pos- 

 sible that a fluid of very low refractive power may exist in the diamond cavi- 

 ties 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. 



The existence of a compressed structure round the cavities clearly proves 

 that the diamond has been in a soft state ; but it may be shown, from various 

 considerations, that this softness was not the softness produced by igneous 

 fusion, and that it is likely to have been the softness of a semi-indurated 

 gum. I have already stated that no such cavities exist in minerals of igneous 

 origin ; a fact which entitles us to separate the diamond from that class of 

 crystals : and it is equally important to observe that its polarizing structure, 

 which I have studied with peculiar care in a great variety of specimens, con- 

 nects it closely with amber and indurated gum. From such substances, in- 

 deed, it differs in having a distinct crystalline form ; but in the mineral resin 

 called mellite we have an equally distinct crystalline form, though there can 

 be little doubt, both from its composition and its locality, that it derives its 

 origin from the vegetable kingdom. 



