ON THE EFFECTS OF URBAN FOG UPON CULTIVATED PLANTS. 31 



a filter-paper — except in the case of nicotine.* This last was 

 applied by burning tobacco (" coarse-cut Cavendish "), and lead- 

 ing the fumes by a tube into the large bell-glass containing 

 the plants. 



The action of all these substances upon foliage is identical 

 with that of pyridine. Perhaps the most violent plasmolysis 

 was produced by thiophene, a substance in which two of the C H 

 groups are replaced by an atom of S. The Bouvardia buds were 

 turned brown by picoline and lutidine — the only representatives 

 of the series (except pyridine) to which they were exposed. 



The above results are of interest in that they show, beyond a 

 doubt, the exceedingly poisonous nature of some of the " tarry 

 products." Thus sulphurous acid is by no means the only 

 substance present in fog injurious to vegetation. Though their 

 toxic character may have been suspected (c/. p. 6), I am not 

 aware that it has ever been demonstrated before. Another point 

 should not be forgotten. These bodies, compared with sul- 

 phurous acid, have little action on the chlorophyll. Now in 

 several instances of foliage injured by fog the extract showed 

 that the chlorophyll was not affected. Some time ago,f 

 specimens A, B, C, being three batches of fog-injured leaves, 

 were submitted to Dr. Schunck, and his account I now quote from 

 his letter to me at the time. " I examined the leaves," he says, 

 " separating the parts that had turned brown from those that 

 were still green as well as I could, extracting each lot with boil- 

 ing alcohol as usual. The two liquids could not be distinguished, 

 both having the usual bright green colour and showing the 

 absorption bands of chlorophyll. I quite expected that the 

 extracts of the discoloured parts of the leaves would have shown 

 on spectroscopic examination that those portions had been 

 exposed to the action of an acid of some kind, but this was not 

 the case. The extracts A and C did, however, show indications 

 of the presence of acid of some kind ; but in the case of C, this 

 might have been due to the previous treatment with boiling 

 water, by which the chlorophyll was slightly changed. When 

 leaves are discoloured, as these were, this is not necessarily due 

 to acid ; there may be other things present in fog besides acid 



* The well-known phenomenon in horticultural practice of injury to 

 vegetation through over-fumigation with tobacco is but a special instance 

 of the deleterious action of such substances as are here under discussion. 



f Before I had made any experiments with the tarry matters. 



