Of the FLEXIBILlTr of Sec. 8y 



the two ends in a horizontal pofition, the middle part bends by 

 its own weight more than a quarter of an inch from the ftraight 

 line. This fpecies of flexibility may certainly be made a pro- 

 per object of fcientifical inveftigation. I am therefore induced 

 to lay before this Society what has occurred to me upon the 

 fubject. 



Hard bodies are either on the one hand friable, or on the 

 other ductile. If they are friable, they are elaflic ; if ductile, 

 again, they preferve the change which has been forcibly induced 

 upon their form, confequently are not in that fenfe elafhic. Bo- 

 dies, indeed, may be either friable or ductile in various degrees ; 

 but, fo far as friable, they are not ductile ; and, fo far as duc- 

 tile, they cannot be faid to be elaflic. But compound bodies 

 may be flexible, without being either ductile or elaflic ; fuch 

 are jointed bodies. In that cafe, however, it is not to the na- 

 ture of the fubftance that the body owes its flexibility, but 

 more properly to its mechanical conflruction. Of this kind, 

 certainly, is the body which we have now under confideration ; 

 for it has a certain flexibility, to which neither the terms duflile 

 nor elajiic, will properly apply ; although, having no degree of 

 ductility from the nature of its fubftance, it cannot, in like 

 manner, be faid to have no elaflicity. The flexibility of this 

 flone is fo eafy, compared with the rigidity of its fubftance, 

 and its elaflicity fo fmall, compared with its flexibility, that 

 there muft be in this body fome mechanical flructure, by which 

 this unnatural degree of flexibility is produced ', that is to fay, 

 a flexibility which is not inherent in the general fubftance of the 

 body. 



Now, the fubftance of this flone being chiefly quartz, the 

 mofl rigid and inflexible of all materials, and the flone, at the 

 fame time, bending in fuch an eafy manner, there is reafon ta- 

 conclude, that this arifes from no principle of flexibility in the 

 general^ fubftance of the flone, but from fome fpecies of articula- 

 tion in the flructure of it, or among its conftituent parts ; which 



articulation 



