Of the FLEXIBILITY of, &c. | 87 
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. , 
Harp bodies are either on the one hand friable, or on the 
other ductile. If they are friable, they are elaftic; if dudtile, 
again, they preferve the change which has been forcibly induced 
upon their form, confequently are not in that fenfe elaftic. 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 elaftic. But compound bodies 
may be flexible, without being either dutile or elaftic; 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 conftruction. Of this kind, 
certainly, is the body which we have now under confideration 3 
for it has a certain flexibility, to which neither the terms ductile 
nor ¢/a/iic, 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 elafticity. The flexibility of. this 
ftone is fo eafy, compared with the rigidity of its fubftance, 
and its elafticity fo {mall, compared with its flexibility, that 
there muft be in this body fome mechanical flruture, 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 ftone being chiefly quartz, the 
mott rigid and inflexible of all materials, and the ftone, at the 
fame time, bending in fuch an eafy manner, there is reafon to: 
conclude, that this arifes from no principle of flexibility in the 
general {ubftance of the ftone, but from fome fpecies of articula- 
tion in the ftructure of it, or among its conftituent parts ; which 
articulation 
