THE CORROSION OF BUILDING STONES. 309 



and sulphur. When heated the sulphur is separated and 

 burns, producing sulphurous acid, which, exposed to moist 

 air, gradually takes up more oxygen and becomes sulphuric 

 acid, which in concentrated solution is oil of vitriol. In 

 the air it is very much diluted by diffusion, but is still 

 strong enough to do mischief to some kinds of building 

 materials. 



In manufacturing towns, such as Birmingham and Shef- 

 field, the quantity of this acid in the air is much greater 

 than in London, and there its mischief is consequently 

 more distinctly visible. The church of St. Philip, which 

 stands nearly in the middle of Birmingham, and is sur- 

 rounded by an old churchyard, was so corroded by this acid 

 that the stone peeled away on all sides, and its condition 

 was most deplorable. The tombstones were similarly dis- 

 integrated 011 their surfaces, and inscriptions quite obliter- 

 ated. It became so bad that a few years ago restoration 

 was necessary, and it was newly faced accordingly. 



Some of the old tombstones that are preserved may still 

 be seen against the church wall, and their peculiar structure 

 is well worthy of study. They display a lamination or 

 peeling away due to unequal corrosion, certain layers of the 

 material of the stone having been evidently eaten away 

 more rapidly than others. Anybody visiting Birmingham 

 may easily examine these, as St. Philip's churchyard is 

 situated between the two railway stations of New Street 

 and Snow Hill, and is but two minutes' walk from 

 either. 



Other stone buildings in the town have suffered, but in 

 very different degrees, and some have quite escaped, prov- 

 ing the necessity of careful selection of material wherever 

 coal fires abound. In Birmingham the action of coal fires 

 is assisted by other sources of acid vapor. The process of 

 "pickling" brass castings, i.e., brightening their surface, 

 by dipping first in common nitric acid ("'pickle acky") 

 and then in water, is attended with considerable evolution 

 of acid fumes. Besides this very widespread use of acid, 

 there are several chemical manufactories that throw still 

 more acid into the air immediately surrounding them. 



As an example of the action of the atmospheric acids of 



