ON THE EFFECTS OF URBAN FOG UPON CULTIVATED PLANTS. 3 



present in the air in foggy weather. An enormous number of 

 different substances are discharged into the air in the destruc- 

 tive distillation of coal, but it is only a few of these that can be 

 actually detected and demonstrated to exist in smoke. Good 

 work has been done in this department by the late Angus Smith 

 and by Dr. W. J. Russell ; whilst latterly Dr. G. H. Bailey, of 

 the Owens College, Manchester, has carried out the most 

 systematic analyses of the air of his own town that have ever 

 been attempted. But all these observers have dealt with those 

 substances, like ammonia, carbonic acid, sulphurous and sul- 

 phuric acids, hydrochloric acid, &c, which are relatively easy of 

 detection, and the fluctuating amounts of which in the air they 

 have determined. But of the organic matters, distilled from 

 coal, which are present in the air there is little certain informa- 

 tion, except as to their aggregate amounts. The organic matter 

 which contributed 14*3 per cent, of the deposit left by the fog 

 upon the glass roofs of Orchid-houses at Chelsea * is composed 

 of hydrocarbons and their derivatives (12*3) and organic bases 

 (2-0). Amongst the unspecified hydrocarbons are no doubt many 

 of the less volatile tarry substances such as anthracene, naphtha- 

 lene, and the phenols. The organic bases will include bodies of 

 the pyridine series. It would be interesting to know what may be 

 the fate of the more volatile hydrocarbons which must pass into 

 the air. Are they represented in these deposits in some altered 

 form ? And do the less volatile ones assist in their precipita- 

 tion — acting as collectors, as has been suggested ? Still, even 

 the fact that such bodies are present in large quantity in fog is 

 exceedingly interesting. For I have been able to show that 

 many of them are, even in small quantities, most injurious to 

 vegetation. But the details of this investigation will appear 

 below. I cannot state the circumstances more tersely than has 

 been done by Dr. Hugo Miiller, F.R.S., a chemist of great ex- 

 perience, in a letter addressed to me.f He says : "I have 

 become more and more convinced that, apart from the identifica- 

 tion of the sulphurous acid, it will be next to impossible to 

 obtain exact chemical evidence of the presence of the other 

 suspected obnoxious substances present in fog, unless very large 

 quantities of fog-washings can be dealt with. Indeed, the minute- 

 ness of the quantity of the deleterious substances, sufficient to 

 * February 1891. f February 16, 1891. 



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