of Edinburgh, Session 1879 - 80 . 523 
blisters which, when they break, expose the interior to rapid dis- 
integration. 
So long as this begrimed film lasts unbroken, the smooth face of 
the marble slab remains with apparently little modification. The 
inscription may be perfectly legible. The moment the crust is 
broken up, however, the decay of the stone is rapid. For we then 
see that the cohesion of the individual crystalline granules of the 
marble has already been destroyed, and that the merest touch causes 
them to crumble into a loose sand. 
It appears, therefore, that two changes take place in upright 
marble slabs freely exposed to rain in our burial-grounds — a super- 
ficial, more or less firm crust is formed, and the cohesion of the 
particles beneath is destroyed. 
The crust varies in colour from a dirty grey to a deep brown 
black, and in thickness from that of writing paper up to sometimes 
at least a millimetre. One of the most characteristic examples of it 
was obtained from an utterly decayed tomb (erected in the year 
1792) on the east side of Canongate Churchyard. No one would 
suppose that the pieces of flat dark stone lying there on the sand- 
stone plinth were once portions of white marble. Yet a mere touch 
suffices to break the black crust, and the stone at once crumbles to 
powder. Nevertheless the two opposite faces of the original polished 
slab have been preserved, and I even found the sharply chiselled 
socket-hole of one of the retaining nails. The specimen was care- 
fully removed, and soaked in a solution of gum, so as to preserve it 
from disintegration. On submitting the crust of this marble to micro- 
scopic investigation I found it to consist of particles of coal, grains 
of quartz-sand, angular pieces of broken glass, fragments of red brick 
or tile, and organic fibres. This miscellaneous collection of town 
dust was held together by some amorphous cement, which was not 
dissolved by hydrochloric acid. At my request my friend Mr B. N. 
Peach tested it with soda on charcoal, and at once obtained a strong 
sulphur reaction. There can be little doubt that it is mainly sulphate 
of lime. The crust which forms upon our marble tombstones is thus 
a product of the reaction of the sulphuric acid of the town-rain 
upon the carbonate of lime. A pellicle of amorphous gypsum is 
deposited upon the marble, and encloses the particles of dust which 
give the characteristic sooty aspect to the stone. This pellicle, of 
