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degrees, blackness by exposure to the air. He shewed some 
reasons for doubting if this was due simply to the effects of 
smoke, and shewed that in all cases, but more markedly in the 
Portland than in other stones, the blackness was either pre- 
vented or removed by the incidence of the sun’s rays. Many 
eases of this were adduced from the buildings in the University, 
and the black and white portions of St Paul’s Cathedral were 
shewn to be referable to the same causes, or apparently to fol- 
low the same law. That the black was not due solely to smut 
or smoke-marks, was inferred from portions scraped from the 
blackened surfaces being found to be quite unaffected by soap 
or solution of soda, and presenting a changed appearance under 
the microscope. It was suggested, as a question of scientific in- 
terest, that the potash or phosphate of lime in some of these 
stones might, in the course of years, undergo some chemical 
change analogous to oxidisation ; at all events, difficulties were 
pointed out in the common and obvious conclusion, that the 
blackening of buildings was in all cases due to the effects of 
smoke alone. | 
Mr TrorreR made some enquiries as to the positions of the 
blackened surfaces with reference to the channels down which 
rain might run. 
Mr O. FisHEeR had only seen these blackened surfaces in 
London and Cambridge; and that the black was removed by 
slight exfoliation of the stone when frozen. The stone which 
lay about near the quarry was not blackened. Possibly the 
“quarry water” might have something to do with it. 
Professor MILLER thought that some kind of vegetable 
growth was the chief cause, instancing the black stains common 
on limestone and dolomite cliffs. The red sandstone of Stras- 
burg Cathedral—though apparently not a favourable stone— 
was covered with vegetation. 
Professor LIvEING mentioned the blackness on parts of the 
white marble in the floor of King’s College Chapel. This he 
