698 



PROTECTION AGAINST DISEASES. 



forming dense clouds which, after escaping from alkali- works, 

 settle on fields and kill whole patches of the crops in them ; 

 it is, however, on the whole less hurtful to woods and crops 

 on a large scale than sulphur dioxide, and the same may be 

 said of the similar action of the oxides of nitrogen, and 

 chlorine. Dr. Angus Smith gives the following comparative 

 statement of acidity of air at different places in England : — 



In a field near Blackpool he found 20"27 grains of hydro- 

 chloric acid and 155'30 grains of sulphur dioxide in 1,000,000 

 cubic feet of air, and the quantities in the other places may 

 be calculated from these figures. 



Dr. Hamburger* states that he exposed leaves to the action of 



11, , , . and ;r-^:zr-, oi sulphuric and hydrochloric 



10 100 1,000 2,000 ^ ^ 



acids of equivalent strength, n being a normal solution of 



49 grammes of sulphuric acid, or 36 J grammes of hydrochloric 



acid in one litre of water. 



The normal solution produced discoloration in about half an 



hour, yellowish-brown spots appearing in the middle of the leaves 



and extending gradually over the whole surface. The — took 



3 hours before signs of destruction appeared. The 

 acted in about a day, but the action of the sulphuric acid was 

 stronger than that of hydrochloric acid. The ^—-sulphuric 



acid produced discoloration in about a week, while the 

 hydrochloric acid required 10 days to do so. 



* "Journal of the .Society of Chemical Industry,'' 1884, p. 205, 



100 



