658 THE POPULAR SCIENCE MONTHLY. 



of particles of coal, grains of quartz-sand, angular pieces of broken 

 glass, fragments of red brick or tile, and organic fibers. This miscel- 

 laneous 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 amor- 

 phous gypsum is deposited upon the marble and incloses the particles 

 of dust which give the characteristic sooty aspect to the stone. This 

 pellicle, of course, when once formed, is comparatively little affected 

 by the chemical activity of rain-water. Hence the conservation of the 

 even surface of the marble. It is liable, however, to be cracked by an 

 internal expansion of the stone to which I shall immediately refer, and 

 also to rise in small blisters, and, as I have said, its rupture leads at 

 once to the rapid disintegration of the monument. 



The cause of this disintegration is the next point for consideration. 

 Chemical examination revealed the presence of a slight amount of sul- 

 phate in the heart of the crumbling marble ; but the quantity appeared 

 to me to be too small seriously to affect the cohesion of the stone. I 

 submitted to microscopic examination a portion of a crumbling urn of 

 white marble in Canongate Churchyard. The tomb bears a perfectly 

 fresh date of " 1792 " cut in sandstone over the top ; but the marble 

 portions are crumbling into sand, though the structure faces the east? 

 and is protected from vertical rain by arching mason-work. A small 

 portion of the marble retaining its crust was boiled in Canada balsam, 

 and was then sliced at right angles to its original polished surface. By 

 this means a section of the crumbled marble was obtained which could 

 be compared with one of the perfectly fresh stone. From the dark 

 outer amorphous crust with the carbonaceous and other miscellane- 

 ous particles, fine rifts could be seen passing down between the sepa- 

 rated calcite granules, which in many cases were quite isolated. The 

 black crust descends into these rifts, and likewise passes along the 

 cleavage-planes of the granules. Toward the outer surface of the 

 stone, immediately beneath the crust, the fissures are chiefly filled 

 with a yellowish, structureless substance, which gave a feeble glim- 

 mering reaction with polarized light, and inclosed minute amorphous 

 aggregates like portions of the crust. It probably consists chiefly of 

 sulphate of lime. But the most remarkable feature in the slide was 

 the way in which the calcite granules had been corroded. Seen with 

 reflected light, they resembled those surfaces of spar which have been 

 placed in weak hydrochloric acid to lay bare inclosed crystals and 

 zeolites. The solution had taken place partly along the outer surfaces, 

 so as to produce the fine passages or rifts, and partly along the cleav- 

 age. Deep cavities, defined by intersecting cleavage-planes, appeared 



