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At other times, wood thus incrufted (hall be eroded bv 
the matter which covers it, having fomething acri- 
monious in its fubflance. We may add to thefe, clus- 
ters of the twigs of fhrubs, and Small wood, which 
we find flakes of, incrufted with fparry or calcarious 
matter, in many places ; parts of which are totally 
changed into that matter, whilft others are only in- 
veloped with it. 
Bones of Animals. 
We fee, by every day’s experience, that the human 
Skeleton moulders to duff in a very few years, when 
buried in mould : fo it does even in vaults, where 
the coffins are kept dry. In the firft cafe, the moif- 
ture and Salts of the earth divide and diffolve the tex- 
ture of the bones ; in the latter, thofe of the air, 
which gradually infinuate themfelves into them, and 
at length deftroy them. How long a fkeleton whofe 
bones are well dried and prepared, being totally de- 
prived of its medullary fubflance, will laft, as we now 
order them for anatomical purpofes, we cannot fay : 
but it may be reafonably conjectured, that they will 
undergo the fate of the Softer kinds of wood, Such 
as beech, which grows rotten in no great number of 
years ; becaufe their internal fubflance is Spungy and 
cellular, and their cruft is very thin, except about the 
middle of the bones of the arm and thigh, I mean the 
humerus and fcemur. The fame deftruCtion would 
happen, if bodies were depofited in a Sandy foil j be- 
caufe water finds its way either by dripping downwards, 
or by Springs underneath. But human Skeletons have 
been found intire within a rock, where neither inoif- 
ture 
