CHAPTEE IV. 



SULPHUROUS ACID— SULPHURIC ACID (OIL OF VITRIOL)— CHLORINE 

 —HYDROCHLORIC ACID— NITRIC ACID. 



6. Sulphurous Acid, SO.. — Preparation.— This gas is prepared 



on the large scale hy the oxidation of sulphur. When sulphur is 

 inflamed in contact with air this product burns and is converted into 

 sulphurous acid. Pyrites, or sulphide of iron, which is roasted at a 

 great heat, may also be used to obtain this same acid. That is the 

 process most usually employed. The gas is led into a gasometer, 

 where it is liquefied by cooling. 



Properties. — Sulphurous acid is colourless and very fluid. It boils 

 at 8° C. below zero ; its density is 1'4:5. Sulphurous acid is not com- 

 bustible. Its use is, therefore, attended wnth no risk of fire. Respired 

 in large quantities it induces suffocation, but in small quantities it 

 is not injurious to man. 



Action of Sulphurous Acid on Plants. — All plants, with- 

 out exception, suffer in an atmosphere of SO^. The leaves absorb 

 this gas, very much attracted by water, and hydrate it. Almost all 

 the sulphurous acid is retained there, and they are the first to suffer. 

 A very small quantity then passes into the wood of the plant. Under 

 the influence of the solar radiation the hydrated sulphurous acid is 

 transformed into sulphuric acid, with liberation of sulphur, according 

 to Loew's equation : — 



The numerous analyses made on dead plants, after the absorption of 

 sulphurous acid, all, in fact, give abnormal quantities of sulphuric 

 acid. 



Wislizenus, who widened the question by very conclusive experi- 

 ments, believes that sulphurous acid, absorbed by the plant, has no in- 

 jurious action on the latter, except the plant be full in the light, and 

 especially in full sunlight ; in darkness the action is nil, although there 

 is absorption of the gas, and in winter the action is very attenuated. 

 According to the experiments, an atmosphere containing a proportion of 

 1 (J0WT70 of its volume of sulphurous acid kills fir-trees (sajnns) in about 

 four weeks if they are in contact with this air in full daylight, but these 

 same firs do not appear to suffer if the experiment be repeated in 

 darkness, even if the atmosphere contain -oy^ooo ^^ sulphurous acid. 

 It is thus evident that sulphurous acid is only converted into sulphuric 



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