282 SCIENCE IN SHORT CHAPTERS. 



Coining as they did from Paris, many supposed that this was one of 

 the latest Paris fashions, and the dandies of the period narrowly 

 escaped the compulsion to follow it. They probably would have 

 done so had the Frenchmen shown any attempt at decorative shaping 

 of the paper. They neglected this because it was litnrus paper, and 

 their object in attaching it to their hats was to test the impurities of 

 the London atmosphere. 



Blue litmus paper, as everybody knows nowadays, turns red when 

 exposed to an acid. The French chemists found that their hat-deco- 

 rations changed color, and indicated the presence of acid in the air 

 of London ; but when they left the metropolis and wandered in the 

 open fields their blue litmus paper retained its original color. By 

 using alkaline paper they contrived to collect enough of the acid to 

 test its composition. They found it to be the acid which is formed 

 by the burning of sulphur, and attributed its existence to the sulphur 

 of our coal. At this time the domestic use of coal was scarcely 

 known in Paris. 



Subsequent experiments have proved that they were right ; that 

 the air of London contains a very practical quantity of sulphurous 

 and sulphuric acids, which are due to the combustion of that yellow 

 shining material more or less visible in most kinds of coal, and has 

 been occasionally supposed to be gold. It is iron pyrites, a com- 

 pound of iron and sulphur. When heated the sulphur is separated 

 aad 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 Sheffield, 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 Birming- 

 ham, and is surrounded 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 disintegrated 

 on their surfaces, and inscriptions quite obliterated. 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 evi- 

 dently eaten away more rapidly than others. Anybody visiting Bir- 

 mingham 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 differ- 

 ent degrees, and some have quite escaped, proving the necessity of 

 careful selection of material wherever coal fires abound. In Birming- 

 ham the action of coal fires is assisted by other sources of acid vapor. 

 The process of " pickling" brass castings i.e., brightening their sur- 

 face by dipping first in common nitric acid (" pickle acky") and then 

 in water, is attended with considerable evolution of acid fumes. Be- 

 eides this very widespread use of acid, there are several chemical 



