140 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



more serious, a wholesale annihilation of the foliage of most 

 tender, soft-wooded stove-plants. The leaves of certain genera 

 of Orchids and of hard-wooded plants turn yellow when these 

 visitations are prolonged. 



It seemed to the Scientific Committee desirable that steps 

 should be taken to ascertain to what constituents of fogs the 

 various classes of injury enumerated above are due ; what part 

 is played by the various acids present ; whether the tarry products 

 exert any specific action ; and how far the conditions of semi- 

 darkness participate in the destruction. 



It was also considered desirable to make special investigation 

 into the exact nature and amount of the impurities present in 

 urban fog, and to observe the varying effects on vegetation of 

 fogs differing in quality. The Committee, though hardly sanguine 

 that any knowledge that might be obtained by such an investiga- 

 tion would lead to any effective abolition of the evil, was of the 

 opinion that such an inquiry was desirable in view of the great 

 interest of the question. In any case, such knowledge was 

 necessary before special cultural precautions could be recom- 

 mended as likely to mitigate the evil to an appreciable extent. 

 A full understanding of the disease must precede any steps for 

 its amelioration. Whilst the Scientific Committee was arranging 

 for the carrying out of a systematic inquiry in the London area, 

 the Manchester Field Naturalists' Society was taking steps in a 

 similar direction. The Manchester Committee proposed to make 

 systematic analyses of fogs at many stations and at various 

 elevations, with a view to finding out everything appertaining to 

 the genesis and composition of their city fogs, with special 

 reference to their injurious effects on animal and vegetable 

 organisms. Further reference will be found in the body of this 

 report to the Manchester investigation. 



Since October last the London inquiry has been in active 

 operation. Circumstances have led the Committee to look tc me 

 for the execution of this research hitherto, and I now summarize 

 shortly the general lines along which the inquiry has pro- 

 ceeded. Throughout, I have had the constant advice and help 

 of my colleagues on the Committee, and my task, without their 

 many suggestions, would have been a more difficult one than it 

 actually has been. It is not proposed to enter here into the 

 accumulations of facts bearing on the question that I have been 



