Jtme 3, 1880] 



NATURE 



107 



amount of curvature from the original vertical line is fully 

 ■z\ inches, so that the hand and arm can be inserted 

 between the curved marble and the perfectly vertical and 

 undisturbed wall to which it was fixed. At the lower 

 end of this slab a minor cui-vature, to the extent of \ inch, 

 is obser\'able coincident with the longer axes of the stone. 

 In both cases the direction of the bending has been deter- 

 mined by the position of the inclosing solid frame of 

 sandstone which resisted the internal expansion of the 

 marble. Freed from its fastenings at either side, the 

 stone has assumed a simple wave-like curve. But the 

 tension has become so great that a series of rents has 

 appeared along the crest of the fold. One of these has 

 a breadth of ^ inch at its opening.' Not only has the 

 slab been ruptured, but its crust has likewise yielded to 

 the strain, and has broken up into a network of cracks, 

 and some of the isolated portions are beginning to curl 

 up at the edges, exposing the crumbling, decayed marble 

 below. I should add that such has been the expansive 

 force of the marble that the part of the sandstone block 

 in the upper part of the frame exposed to the direct 

 pressure has begun to exfoliate, though elsewhere the 

 stone is ciuite sound. 



More advanced stages of curvature and fracture may 

 be noticed on many other tombstones in the same burying- 

 place. One of the most conspicuous of these has a 

 peculiar interest from the fact that it occurs on the tablet 

 erected to the memory of one of the most illustrious dead 

 whose dust lies within the precincts of the Greyfriars — 

 the great Joseph Black. He died in 1799. In the centre 

 of the sumptuous tomb raised over his grave is inserted 

 a large upright slab of white marble, which, facing south, 

 is protected from the weather partly by heavy over- 

 hanging masonry, and partly by a high stone wall imme- 

 diately to the west. On this slab a Latin inscription 

 records with pious reverence the genius and achievements 

 of the discoverer of carbonic acid and latent heat, and 

 adds that his friends wished to mark his resting-place 

 by the marble whilst it should last. Less than eighty 

 years, however, have sufficed to render the inscription 

 already partly illegible. The stone, still firmly held all 

 round its margin, has bulged out considerably in the centre, 

 and on the blister-like expansion has been rent by 

 numerous cracks which run on the whole in the direction 

 of the length of the stone. 



A further stage of decay is exhibited by a remarkable 

 tomb on the west wall of the Greyfriars Churchyard. The 

 rnarble slab, bearing a now almost wholly effaced inscrip- 

 tion, on which the date 1779 can be seen, is still held 

 tightly within its inclosing frame of sandstone slabs, 

 ■which are firmly built into the wall. But it has swollen 

 out into a ghastly protuberance in the centre, and is more- 

 over seamed with rents which strike inwards from the 

 margins. In this and in some other examples the marble 

 seems to have undergone most change on the top of the 

 swelling, partly from the system of fine fissures by which 

 it is broken up, and partly from more direct and effective 

 access of rain. Eventually the cohesion of the stone at 

 that part is destroyed, and the crumbling marble falls out, 

 leaving a hole in the middle of the slab. When this 

 takes place disintegration proceeds rapidly. Three years 

 ago I sketched a tomb in this stage on the cast wall of 

 Canongate Churchyard. In a recent visit to the place I 

 found that the whole of the marble had since fallen out. 



The first cause that naturally suggests itself in ex- 

 planation of this remarkable change in the structure of a 

 substance usually regarded as so inelastic is the action of 

 frost. White statuary marble is naturally porous. It is 

 rendered still more so by that internal solution which I 

 have described. The marble tombstones in our grave- 

 yards are therefore capable of imbibing a relatively large 



' It is a further curious fact that the flab measures half an inch more in 

 breadth across the centre where it has had room to expand than at the top 

 where it has been tightly jammed between the sandstone slabs.. 



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 transformation 

 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 

 g;rounds show that, save in exceptionally sheltered, situa- 

 tions, 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. When 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 nine millimetres) in a century. Where, 

 it is effected by internal displacement, a curvature of two 

 and a half 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 disruption and effacement of the stone 

 within one hundred. It is evident that white marble is- 

 here utterly unsuitei 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 

 strenuously 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. 



II. S.\NDSTONES AND FLAGSTONES.— These, being the 

 common building materials of the country, are of most 

 frequent occurrence as monumental stones. When pro- 

 perly selected, they are remarkably durable. By far the 

 best varieties are those which consist of a nearly pure fine 

 siliceous sand, with little or no iron or lime, and without 

 trace of bedding structure. Some of our sandstones con- 

 tain 98 per cent, of silica. A good illustration of their 

 power of resisting the weather is supplied by Alexander 

 Henderson's tomb in Greyfriars Churchyard. He died 

 in 1646, and a few yeirs afterwards the present tomb- 

 stone, in the form of a solid square block of freestone, was 

 erected at his grave. It was ordered to be defaced in 

 1662 by command of the Scottish Parliament, but after 

 1 688 it was repaired. Certain bullet marks upon the 

 stone are pointed out as those of the soldier)' sent to 

 execute the order. Be this as it may, the original chisel 

 marks on the polished surface of the stone are still per- 

 fectly distinct, and the incised lettering remains quite 

 sharp. Two hundred years have effected hardly any 

 change upon the stone, save that on the west and south 

 sides, which are those most exposed to wind and rain, the 

 surface is somewhat roughened, and an internal fine 

 parallel jointing begins to show itself. 



Three obvious causes of decay in arenaceous rocks 

 may be traced among our monuments. In the first place, 

 the presence of a soluble or easily removable matrix ia 

 which the sand grains are embedded. The most 

 common kinds of matrix are clay, carbonates of lime and 

 iron, and the anhydrous and hydrous peroxides of iron. 

 The presence of the iron reveals itself by its yellow, 

 brown, or red colour. .So rapid is disintegration from 

 this cause, that the sharply- incised date of a monument 



