┌──────────────────────────────────────────┬─────────────────────────┐ │ │VOLUMES OF CARBONIC ACID │ │ │IN 10,000 VOLUMES OF AIR.│ │ └─┬───────────────────────┤ │_On the mountains and moors of Scotland │ │ │ ──mean of 57 analyses_ │ 3·36 │ │_In the streets of Glasgow │ │ │ ──mean of 42 analyses_ │ 5·02 │ │_London, N., N.E., and N.W. postal districts│ │ │ ──mean of 30 analyses_ │ 4·384 │ │_London, E. and E.C.──mean of 12 analyses_ │ 4·745 │ │_Manchester streets, ordinary weather_ │ 4·03 │ │_During fogs in Manchester_ │ 6·79 │ └────────────────────────────────────────────┴───────────────────────┘ The effects of carbonic acid gas alone must be carefully distinguished from those of the same gas _plus_ the organic impurities from respiration, with which it is commonly associated. Dr. Angus Smith found that air containing 3 per cent. of carbonic acid produced difficulty of breathing, but he was able to breathe comfortably an atmosphere containing 0·2 per cent. carbonic acid. Other observers have found they could breathe without discomfort air containing 1 per cent. carbonic acid. When the carbonic acid is derived from respiration, headache and giddiness are produced in many persons when the carbonic acid amounts to 0·15 per cent. A fatal result has occasionally occurred from the inhalation of the carbonic acid at the bottom of brewing vats, or about lime-kilns. The gaseous impurity at the bottom of wells is more commonly nitrogen than carbonic acid (page 102). The presence of an excess of carbonic acid diminishes the elimination of carbonic acid from the lungs, nutrition and muscular energy being consequently impaired. This is seen in workshops where the air is confined and gaslight is commonly employed; though the air here also contains carbonic oxide, sulphurous acid, and organic impurities, and these probably have a large share in producing the evil results.