ON THE EFFECTS OF URBAN FOG UPON CULTIVATED PLANTS. 57 



strength that the aspiration of 2^ to 3 cubic feet of an ordinary fog 

 would decolorise the solution, whilst 1^ to 2 cubic feet sufficed in 

 the case of very severe fogs. I have repeatedly aspirated air, in all 

 sorts of foggy weather, through the charcoal-box. But even in 

 the most severe instances I have never noticed anything more 

 than a slight discoloration of the permanganate after the passage 

 of as much as 25 cubic feet. I have also placed the box in a 

 chamber into which an atmosphere of strong sulphurous acid 

 was introduced — an atmosphere of which T } {1 cubic foot sufficed to 

 entirely decolorise the permanganate. When drawn through the 

 charcoal, however, 3 cubic feet could be drawn without perceptibly 

 affecting the colour of the fluid. When kept in an atmosphere 

 of strong sulphurous acid the charcoal becomes in time charged, 

 and, for the time being, incapable of further absorption. In this 

 charged condition I left the box for some eight or ten weeks, and 

 found that by the expiration of that time it was as good an 

 absorber as ever. With ordinary fogs there seems little fear of 

 anything of this kind happening; nor have I observed any ten- 

 dency in the charcoal to get choked in this way in long spells of 

 foggy weather. That other impurities are also absorbed I have 

 no proof, though I consider it most probable. 



In order to demonstrate the advantages of his system to 

 horticulturists, Mr. Toope has constructed a small conservatory 

 at his offices in Stepney. Here he cultivates, in an unfavourable 

 atmospheric environment, a collection of Orchids and other stove 

 plants. The results I regard as distinctly favourable to his 

 system, though they were not by any means convincing. This 

 arose, not necessarily from any defect in the filtering apparatus, 

 but rather from faulty cultural methods. Mr. Toope is a busy 

 man, and the charge of his plants falls to the lot of others. 

 Many plants very sensitive to atmospheric impurities, which he 

 obtained at my suggestion, received a severe check in transit 

 before they reached him. Others, again, which he raised from 

 -seed for observation were liable to neglect from time to time. So 

 that a casual visitor unacquainted with the facts might easily 

 have carried away an unfavourable impression of the utility of 

 the system. But, taking everything into consideration, I incline 

 jfco take a distinctly favourable view of charcoal as a filter for 

 contaminated air — so much so that I believe it might be adopted 

 with advantage by our urban cultivators. The charcoal un- 



