230 Memoir on the Foffil Caout-Choue, 



Variety IV. Foffil caoat-choiic, compreffible, and having a 

 relation to the firft variety, but of"a darker colour, adhering 

 to gray calcareous fpar mixed with fome grains of galena. 



yar'teiy V. Caout-chouc of a liver brown colour, much 

 lefs unttuous to the touch than the firlt variety, but com- 

 preffible, and having the afpeCt of the real natural elaftic 

 gum; and the more remarkable, as it is found to become 

 iolid,as we may lay, in fome parts, where it acquires a much 

 greater hardnefs, and even becomes brittle, and acquires a 

 vitreous fplendour. This gradual change is fo ftriking, that 

 we cannot confider this hard bitumen, which in the prefent 

 fpecimen is of a yellowiih colour, as a hard bituminous jnatter 

 accidentally enveloped by caout-chouc. 



Section II. 



Solid and Brittle Fojjil Caout-Cbouc. 



No. VI. Black folid caout-chouc, hard and brittle like jet, 

 exceedingly brilliant on its fra6lure, which is conchoid, and 

 fometimts finely marked with ftrite difpofed in the fonn of 

 rays, proceeding from the point of frafture, and expanding in 

 the manner of a fan; eleftric by friftion, opake throughout 

 the whole mafs, but pellucid at the edges, and particularlv 

 when viewed in aftrong light: its colour is then a red, almoil 

 as bright and agreeable as that of the hyacinth, and analogous 

 to the colour obferved oil the edges of the elartic caout-chouc 

 of the fecond variety, feen in the light which feems to an- 

 nounce their identity. 



No. VII. Another variety of hard caout-chouc, fimilar to 

 that of No. VI. in hardnefs and fplendour, but which is of 

 a liver brown colour. It is entirely pellucid by the light of 

 a lamp even throughout its whole fubflance, though opake 

 in common day-light, and its colour is then filnilar to that 

 of the hvacinth. 



No. VIII. The fame variety as the above, but of a paler 

 liver colour. It has befides the fame charafters and the 

 fame properties as the other hard and brittle kinds of caout- 

 chouc ; but it is remarkable for ftill adhering to its matrix, 

 confiftingof femi-tranfparent milky calcareous fpar, with thick 

 brilliant laminae of galena. The caout-chouc here not only 

 adheres to the calcareous fpar but to fulphqrated lead, and is 

 intimately mixed with both. 



Mr. Mawe, in his Mineralogy of Derbyfliire, mentions 

 the following verv curious fail : — •" A variety, the only one 

 J polTcfs of the kind, is elaitic bitumen in a petrified marine 

 fhell indofcd iu the rock ;" <ilfo *' another fpecimen po lefs 



