ON THE EFFECTS OF URBAN FOG UPON CULTIVATED PLANTS. 21 



The following were foggy days : — ■ 



Milligrammes of sulphuric 

 Date (1891). Weather. acid per 100 cubic 



feet of air. 



November 24 Slight fog 10-24 



30 Dark and very foggy 17-10 



December 21 Yellow fog 20-52 



22 Dense black fog 39-06 



23 Yellow fog 12-96 



24 Thick yellow fog 20*40 



The ahove may serve as samples. 



The sulphurous acid generated in the combustion of coal, and 

 'discharged into the atmosphere, is readily oxidised into sulphuric 

 acid. That these two substances co-exist in town air there can 

 be no reasonable doubt. But since most of the exact determina- 

 tions of sulphurous acid had been made as sulphuric acid, some 

 •doubt exists as to the relative amounts present at any given time, 

 .and to the rate of natural conversion of the one into the other. 



It may be stated at the outset that some only of the characters 

 of injury to plants by fog can be successfully imitated by exposing 

 growing plants to the action of an atmosphere containing S0 2 in 

 appropriate dilution. The method employed has been to place 

 the plants in closed glazed cases into which the sulphurous acid 

 could be supplied continuously or periodically as circumstances 

 demanded. The cases used were the corners of a greenhouse 

 cut off by means of glazed frames so that all joints, as also the 

 door of the case, were air-tight. The amount of free S0 2 present 

 •CGuld be exactly determined by drawing off known quantities of 

 the air through a tower down which hydrogen peroxide was 

 .always dripping. The amount was then determined exactly as 

 in the case of the fogs just quoted. But most frequently, and 

 for my purpose very convenient, I aspirated the air through 

 25 c.c. of potassium permanganate in a wash-bottle devised for 

 the purpose. The time occupied by the decoloration of the 

 permanganate was a measure of the amount of S0 2 present, 

 sufficiently accurate for the purpose. Knowing the rate at 

 wiiich the aspirator w T orked, I could then always determine the 

 number of cubic feet of air that it was necessary to aspirate 

 ,to obtain decoloration. I could thus obtain and maintain a 

 concentration of S0 2 in the chamber so that it never exceeded 

 any desired limit. The chambers possessed a capacity of about 

 40 cubic feet. In addition I used large bell-glasses of 6 cubic 

 ieet capacity, the contents of which were similarly under control. 



