20 



JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



II. The Action of Acid and other Vapours. 



Sulphurous Acid. 



That constituent of urban fog, and of coal smoke generally, 

 to which most attention has been directed is sulphurous acid. 

 The injurious action of this substance upon vegetation has been 

 long recognised, and the literature bearing on the question is an 

 extensive one. But it would carry me far beyond the limits of 

 such a report as this were I to enter critically here upon the 

 methods which have been employed and the results which have- 

 been obtained by other investigators. I defer any such exhaustive 

 discussion oi the work of my predecessors till such time as I am 

 in a position to prepare a detailed memoir on the whole subject 

 of the action on plants of the various atmospheric impurities. 

 It is, therefore, deliberately that I make none but occasional 

 references here to previous work. The action of sulphurous- 

 acid is considered first ; afterwards that of certain other sub- 

 stances. 



The amount of this acid present in London air varies from 

 time to time enormously. Whilst in fine weather traces only 

 can be detected, very appreciable quantities are present in dull 

 weather, whilst in characteristic London fogs the amount is 

 greatly increased. A few determinations made during the latter 

 part of 1891 give some indication of the amounts of sulphurous 

 acid present in London air in foggy weather. The method 

 employed is identical with that in use at Manchester ; it is fully 

 described in the " Preliminary Report of the Air Analysis Com- 

 mittee " issued by the Town Gardening Section of the Manchester 

 Field Naturalists' Society, March 1891. Known volumes of air 

 pass through a continuous drip of hydrogen peroxide, which 

 absorbs the sulphurous acid. The determinations are made as 

 sulphuric acid. The locality is University College, Gower Street. 

 The following were simply dull days : — 



Milligrammes of sulphuric 

 Date (1891). Weather. acid per 100 cubic 



feet of air. 



November 5 Dull 5-40 



6 „ 4-73 



10 „ 6-80 



13 „ 5-66 



17 Sliphtfog 8-10 



20 Dull 6-80 



