Temperatures and under Great 'Pressures. 307 



of metal faster than emery powder. It has, under the micro- 

 scope, the appearance of bort, the minute particles seem to 

 cling together ; they are not transparent as a rule, and though 

 some such particles are found among them ; it is not clear that 

 such are hard. 



When a piece of the skin has been rubbed against a 

 diamond or other hard body, the projecting or hard portions 

 have a glossy coke-like appearance. 



A piece of the skin will continue to scratch rock crystal for 

 some time without losing its edge. It will scratch ruby, and 

 when rubbed 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 cut through into the carbon beneath, 

 making a black line or opening about J inch long ; the facet 

 on the diamond, originally -£% 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 

 investigated 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 dis- 

 tinctive features. 



The mode of formation is not inconsistent with the con- 

 ditions 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, 



