308 SCIENCE IN SHORT CHAPTERS. 



Cure must be taken not to confound these with crystals of 

 the salt which may also be deposited. These crystals are 

 easily removed by adding a little more water or warming 

 the solution. 



For strict comparison the fragments thus separated 

 should be weighed in a delicate balance, such as is used in 

 chemical analysis. 



THE COKROSION OF BUILDING STOKES. 



ABOUT fifty years ago two eminent French chemists 

 visited London, and rather "astonished the natives" by a 

 curious feature of their dress. They wore on their hats 

 large patches of colored paper. Coming, 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 decora- 

 tive shaping of the paper. They neglected this because it 

 was litmus 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 now-a-days, turns 

 red when exposed to an acid. The French chemists found 

 that their hat-decorations 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 exist- 

 ence 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 compound of iron 



