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Proceedings of the Royal Society 
naturally porous. It is rendered still more so by that internal 
solution which I have described. The marble tombstones in our 
graveyards are, therefore, capable of imbibing a relatively large 
amount of moisture. When this interstitial water is frozen, its 
expansive force, as it passes into the solid state, must increase the 
isolation of the granules and augment the dimensions of a marble 
block. I am inclined to believe that this must be the principal 
cause of the change. Whatever may be the nature of the process 
it is evidently one which acts from within the marble itself. 
Microscopic examination fails to discover any chemical transforma- 
tion which would account for the expansion. Dr Angus Smith has 
pointed out that in towns the mortar of walls may be observed to 
swell up and lose cohesion from a conversion of its lime into the 
condition of sulphate. I have already mentioned that sulphate 
does exist within the substance of the marble, but that its quantity, 
so far as I have observed, is too small to be taken into account in 
this question. The expansive power is exerted in such a way as 
not sensibly to affect the internal structure and composition of the 
stone. And this I imagine is most probably the work of frost. 
The results of my observations among our burial-grounds show 
that, save in exceptionally sheltered situations, slabs of marble, 
exposed to the weather in such a climate and atmosphere as that of 
Edinburgh, are entirely destroyed in less than a century. Where 
this destruction takes place by simple comparatively rapid superficial 
solution and removal of the stone, the rate of lowering of the surface 
amounts sometimes to about a third of an inch (or roughly 9 milli- 
metres) in a century. Where it is effected by internal displacement, 
a curvature of 2J- inches, with abundant rents, a partial effacement 
of the inscription, and a reduction of the marble to a pulverulent 
condition, may be produced in about forty years, and a total dis- 
ruption and effacement of the stone within one hundred. It is 
evident that white marble is here utterly unsuited for out of door 
use, and that its employment for really fine works of art which are 
meant to stand in the open air in such a climate ought to be strenu- 
ously resisted. Of course I am now referring, not to the durability 
of marble generally, but to its behaviour in a large town with a 
moist climate and plenty of coal-smoke. 
v II. Sandstones and Flagstones.— T hese, being the common 
