1888.] 



Presents. 



323 



for some time against it will wear grooves or facets upon it. When 

 a cut diamond is rubbed on the surface of the skin, it will 0°A' 

 through it into the carbon beneath, making a black line or opening 

 about 5-inch long ; the facet on the diamond, originally g^-inch 

 diameter, will have its corners evenly rounded, and its polished 

 surface reduced to about one-half its original area ; the appearance of 

 the edges is as if they had been rubbed down by a nearly equally 

 hard substance. 



The subject of the last experiment is scarcely sufficiently investi- 

 gated to warrant any definite conclusions. 



The substance in the several ways it has so far been tested seems 

 to possess a hardness of nearly if not quite the first quality. The 

 minuteness of the particles, which appear more or less cemented 

 together, and are less cohesive after the action of acid, make it very 

 difficult to determine their distinctive features. 



The mode of formation is not inconsistent with the conditions of 

 pressure, temperature, and the presence of moisture, lime, silica, and 

 other substances as they appear to have existed in the craters or 

 spouts of the Cape Diamond Mines at some epoch. 



From the few experiments that have been made it appears that at 

 pressures below 3 tons per square inch, the deposit does not possess 

 the same hardness, though somewhat similar in appearance. 



What part the lime and silica play, whether the former only 

 supplies moisture and oxygen which combine with the carbon, or 

 whether the presence of lime is necessary to the action, is not clear. 



We may, however, observe that so far it seems as if the lime and 

 moisture combining with the carbon form a gas or liquid at great 

 pressure, which combining with the silica, forms some compound of 

 lime, silica, and carbon, or perhaps pure carbon only, of great 

 hardness. 



Presents, June 14, 1888. 



Transactions. 



Albany : — New York State Museum of Natural History. Bulletin. 



No. 3. 8vo. Albany 1888. The Museum. 



London: — Photographic Society of Great Britain. Journal and 



Transactions. Yol. XII. No. 8. 8vo. London 1888. 



The Society. 



Royal Institute of British Architects. Journal of Proceedings. 



Vol. IV. No. 15. 4to. London 1888. The Institute. 



Society of Biblical Archaeology. Proceedings. Vol. X. Part 7. 



Svo. London 1888. The Society. 



Manchester : — Geological Society. Transactions. Vol. XIX. 



Parts 18-19. 8vo. Manchester 1888. The Society. 



