598 Professor A. H. Church [April I2 



but water, as the carrier of sulphuric acid, and as the solvent of the 

 sulphates which it helps to form, must not be passed by in silence. 

 Moreover, the condensation of moisture upon the surface of a fresco 

 within a building is most harmful, and, when it cannot be wholly 

 avoided, indicates the desirability of applying some waterproofing 

 material to the surface of the painting. As to soot and tarry 

 matters just named, one has to remember that even the total aboli- 

 tion of smoke from coal would in no way lessen the amount of 

 sulphuric acid produced i a the burning of this fuel. The soot and tar 

 of smoke do indeed adhere to stone and discolour and disfigure it, 

 but, except as carriers of acid, they do not corrode it. Here are 

 some figures * from the analysis of a strange crust attached to the 

 under-surface of the cornice above the colonnade and below the 

 dome of St. Paul's Cathedral. This crust has been slowly formed by 

 the conversion of calcium carbonate from the Portland stone above 

 the cornice into gypsum. This change has been effected by the 

 acid rain of the City ; then the gypsum has entered into solution, 

 which has hung as drip to the lower face of the cornice, until the 

 water has evaporated, and the residual gypsum, dirty with 1 per cent, 

 of soot and somewhat less of tar, has formed a stalactitic mass, 

 occasionally reaching 3 inches in depth. The phosphate of lime 

 present has never been dissolved, but represents one of the con- 

 stituents of the original stone mechanically carried down and 

 entangled in this dark grey stalactitic gypsum. But a visit to 

 St. Paul's is not needed to see what damage is wrought by sul- 

 phuric acid even upon Portland stone. Its decay, being regular 

 and even, often passes unnoticed, but may be watched in all parts 

 of London, in balconies, copings, quoins, etc. 



Allow me to direct your attention to the two chief sources of 

 the sulphuric acid found in the atmosphere of towns, and especially 

 in that of London. There is no doubt that the larger proportion 

 of the sulphur present in coal escapes during burning in the forms 

 of sulphuric and sulphurous acids. Some remains in the ash, and 

 this quantity may be increased by mingling slaked lime with the 

 coal. [Lime-water, which has been suggested for this purpose, is 

 far too weak ; indeed, 3000 gallons, or 18 tons 8 cwt. would be 

 required to fix the sulphur of 1 ton of coal !] But there is a minor 

 though by no means negligible source of sulphuric acid in the pro- 

 ducts of the combustion of coal-gas. Since the restrictions imposed 

 on the gas companies of London have been relaxed — that is, since 



* Incrustation from drip below cornice, St. Paul's : — 



Carbon 1*01 per cent. 



Ammonium sulphate 0*93 ,, 



Tar 0-60 



Gypsum 73-80 



Calcium phosphate 2*22 ,, 



Calcium carbonate none. 



