656 THE POPULAR SCIENCE MONTHLY. 



sents three phases, sometimes to be observed on the same slab, viz., 

 superficial solution, internal disintegration, and curvature with frac- 

 ture. 



1. Superficial solutioti is effected by the carbonic acid and partly 

 by the sulphuric acid of town rain. When the marble is first erected 

 it possesses a well-polished surface capable of affording a distinct 

 reflection of objects placed in front of it. Exposure for not more 

 than a year or two to our prevalent westerly rains sufiices to remove 

 this polish, and to give the surface a rougl\, granular character. The 

 granules Avhich have been cut across or bruised in the cutting and 

 polishing process are first attacked, and removed in solution or drop 

 out of the stone. An obelisk in Greyfriars Churchyard, erected in 

 memory of a lady who died in 1864, has so rough and granular a 

 surface that it might readily be taken for a sandstone. So loosely are 

 the grains held together that a slight motion of the finger will rub 

 them off. In the course of solution and removal, the internal struc- 

 ture of the marble begins to reveal itself. Its harder nests and vein- 

 ings of calcite and other minerals project above the surrounding 

 surface, and may be traced as prominent ribs and excrescences run- 

 ning across the faint or illegible inscriptions. On the other hand, 

 some portions of the marble are more rapidly removed than others. 

 Irregular channels, dependent partly on the direction given to trick- 

 ling rain by the form of the monumental carving, but chiefly on 

 original differences in the internal structure of the stone, are gradu- 

 ally hollowed out. In this way the former artificial surface of the 

 marble disappears, and is changed into one that rather recalls the 

 bare, bleached rocks of some mountain-side. 



The rate at which this transformation takes place seems to depend 

 primarily on the extent to which the marble is exposed to rain. Slabs 

 which have been placed facing to the northeast, and with a sufficiently 

 projecting architrave to keep off much of the rainfall, retain their 

 inscriptions legible for a century or longer. But even in these cases 

 the progress of internal disintegration is distinctly visible. Where the 

 marble has been less screened from rain, the rapidity of waste has been 

 sometimes very marked. A good illustration is supplied by the tablet 



of G G , on the south side of Greyfriars Churchyard, who died 



in 1785.* This monument had become so far decayed as to require 

 restoration in 1803. It is now, and has been for some years, for the 

 most part uttei'ly illegible. The marble has been dissolved away over 

 the center of the slab to a depth of about a quarter of an inch. Yet 

 this monument is by no means in an exposed situation. It faces east- 

 ward in a rather sheltered corner, where, however, the wind eddies in 

 such a way as to throw the rain against the part of the stone which has 

 been most corroded. 



* For obvious reasons I withhold the names carved on the tombstones referred to in 

 this communication. 



