British Association for the Advancement of Science. 39 
The results of this experiment in restoring the diamond to its 
value 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. Th 
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 peculiarity 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 opmion. He had, ina 
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 i 
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 
eolor and polish of surface, the coatings applied may not differ. 
In contending for the necessity of equalizing the coatings com- 
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