CHEMICAL SCIENCE. 241 



the last few years, several chemists have directed their attention 

 to this apparent inconsistency between the organoleptic and physi- 

 cal characters of the air with special reference to the condition of 

 the atmosphere in towns. These researches have related mainly to 

 the quantity of carbonic acid, and other products of combustion, and 

 to the existence of organic matter in suspension. Among the most 

 important are those of Dr. Dundas Thomson and Dr. Angus Smith. 



The percentage of carbonic acid usually existing in the air of Lon- 

 don was found by Dr. Roscoe to be 0.037 per volume, a result not 

 differing materially from those obtained by Dumas and Boussingault 

 in Paris. The analyses on which these are based were made by 

 passing a known volume of air over weighed tubes containing alter- 

 nately pumice-stone steeped in sulphuric acid and potash, a method 

 which leaves nothing to be desired in respect of accuracy. Dr. 

 Smith's estimations of the carbonic acid of the air -of Manchester, 

 made by the same method, give somewhat higher results. He found 

 that on a windy day they averaged from .08 per cent, to .45 per cent., 

 and that on a still day the percentage amounted to .12. When, how- 

 ever, we consider that, although London is the greatest city in the 

 world, Manchester is the largest manufacturing town, and that it is 

 the centre of a manufacturing district comprehending many hundred 

 square miles, over which an atmosphere darkened by smoke perpet- 

 ually hangs, we are not surprised to find that the products of com- 

 bustion exist in larger proportion than in London or Paris. Dr. 

 Smith has calculated, from the quantity of coal burnt in the neigh- 

 borhood of Manchester, that 15,000 tons of carbonic acid must be 

 introduced into the atmosphere daily, without taking into account 

 the quantity expired by man and animals. 



A much more important product of combustion is derived from the 

 oxidation of the sulphur contained in coal, and the introduction 

 thereby into the atmosphere of sulphurous and sulphuric acids. In 

 the researches undertaken by Dr. Thomas during the last epidemic 

 of cholera, which consisted in passing large quantities of the air of 

 London through distilled water, it was found that such air invariably 

 possessed an acid reaction, and that this reaction was due to sul- 

 phuric acid. Dr. Smith has further investigated this question, and 

 has found that in Manchester the acid reaction of the atmosphere is 

 much more constant and intense than in London. Blue litmus paper 

 becomes red in half an hour, and sometimes in ten minutes, when 

 exposed to Manchester rain, and occasionally its acidity is such that 

 a single drop is sufficient to effect the reaction. The actual quantity, 

 however, is exceedingly small ; of a solution containing a thousandth 

 part of its weight of carbonate of soda, quantities varying from ten 

 to fifty grains suffice to neutralize one thousand grains of such rain, 

 and as much cistern water is found to be neutralized by twenty-five 

 grains ; from which results Dr. Smith concludes that the largest 

 quantity of sulphur acids existing in the atmosphere of the town does 

 not exceed 0.004 per cent, by weight, a proportion amounting to not 

 more than a twentieth part of the carbonic acid. As to the share 

 of sulphurous and sulphuric acids respectively in this total, it is of 

 course impossible to arrive at a conclusion ; but considering what we 

 know of the rapidity with which the former is oxidized in the air, it 

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