British Association for the Advancement of Science. 39 



The results of this experiment in restoring the diamond to its 

 vahie as an optical material, in so far as it enables us to cut it in 

 a proper direction, and select proper specimens, and its connection 

 with some delicate researches of Profs. Airy and Maccullagh, on 

 the superficial action of diamond upon polarized light, possess 

 considerable interest, but the fact of a mineral body consisting of 

 layers of different refractive powers, and consequently different 

 degrees of hardness and specific gravity, is remarkable. There 

 were several minerals, such as Apophyllite^ Chabasie, and others, 

 in which Sir David said that he had found different degrees of 

 extraordinary refraction in different parts of the crystal ; but this 

 variation of property depends upon a secondary law of structure • 

 and he believed that there was no crystal, either natural or artifi- 

 cial, in which the properties of ordinary refraction, hardness, and 

 specific gravity, varied throughout its mass. This pecuharity of 

 structure, therefore, might be regarded as an indication of a pecu- 

 liarity of origin ; and as there are various strong arguments in 

 favor of the opinion, that the diamond is a vegetable substance, 

 the new structure which he had described might be considered 

 as an additional argument in favor of that opinion. He had, in a 

 former paper, placed it beyond a doubt, that the diamond must 

 have been in a soft state, like amber or gum, and capable of hav- 

 ing its structure modified by the expansive force of air or gaseous 

 bodies imprisoned in its cavities ; and therefore the fact of its be- 

 ing sometimes composed of strata of different degrees of indura- 

 tion and refractive power, was more likely to have been produced 

 by pressures varying during the formation of the crystal, than by 

 any change in the intensity of the forces of aggregation of its 

 molecules. Such a change might have been supposed probable, 

 had it been found in another crystal. 



Prof Bache on Heat. — The object of this communication is to 

 call the attention of the Section to the researches of Prof Bache 

 of Pennsylvania, which seem not to have been so fully apprecia- 

 ted in this country as they deserve. That gentleman, at the out- 

 set of his inquiries, refers to a paper of Prof Powell, in which the 

 difficulties unavoidably attending any comparison of radiating 

 effects of surfaces are pointed out from the impossibility of deter- 

 mining precisely in how many other respects besides those of 

 color and polish of surface, the coatings applied may not differ. 

 In contending for the necessity of equalizing the coatings com- 



