12 



NATURE 



[November 5, 1891 



to the square mile, and the composition of these deposits 

 is as follows : — 



Carbon 



Hydrocarbons 



Organic bases (pyridines, &c.) 



Sulphuric acid (SO3)... 



Hydrochloric acid (HCi) 



Ammonia ... 



Metallic iron and magnetic oxide 

 of iron... 



Mineral matter (chiefly silica and 

 ferric oxide) 



Water, not determined (say dif- 

 ference) 



These analyses give, I believe, for the first time, a defi- 

 nite account of the composition of fog-deposit. Soot and 

 dust are by far its principal constituents, rendered sticky 

 and coherent by hydrocarbons, but I should like to give 

 you the striking description which Prof. Thiselton Dyer 

 has sent me of this deposit, collected at Kew. He says : 

 " It was like a brown paint, it would not wash off with 

 water, and could only be scraped off with a knife. It 

 thickly coated all the leaves of the evergreens, and upon 

 what have not yet been shed it still remains." In the 

 above analysis it is curious to note the large amount of 

 metallic iron and magnetic oxide of iron. 



The details with regard to these very interesting 

 analyses we shall hear from a member of the Manchester 

 Committee, and I will only ask you to note how large a 

 proportion of these deposits arises from the imperfect 

 combustion of coal. We also learn from the Manchester 

 Committee some interesting facts with regard to fog- 

 deposits which occurred last winter in their city. This 

 deposit which was collected from Aucuba leaves contained 

 as much as 6 to 9 per cent, of sulphuric acid, and 5 to 

 7 per cent, of hydrochloric acid, mostly, of course, in a 

 state of combination, but the deposit was, they say, 

 " actually acid to the taste." Also, that three days' fog 

 deposited per square mile of surface, in by no means the 

 worst part of Manchester, i\ cwt. of sulphuric acid, and 

 even as far out of the city as the Owens College, on the 

 same area, over i cwt. of acid and 13 cwt. of blacks. 



There is still one other point characteristic of town 

 fogs to be noted : it is their persistency in an atmosphere 

 considerably above the dew point. A country fog under 

 such circumstances directly passes away ; a town fog 

 apparently does not do so. There seem to me to be two 

 reasons for this : one is that the moisture is protected, 

 and its evaporation to a large extent hindered, by the 

 presence of oily matter ; and secondly, when the moisture 

 has really gone, the soot and dust remain, and produce a 

 haze. 



The great distance to which fogs will travel is also 

 remarkable, for they have on many occasions been traced 

 to a distance of at least 25 to 35 miles from London, and 

 I believe I might say to 50 miles. 



I have so far discussed the production and compo- 

 sition of town fogs, and before con;idering their effects, 

 would say a word on the question of whether in Lon- 

 don they are increasing in frequency and density. A 

 complete and accurate record of fogs in London is 

 not kept ; several stations are required, and a cor- 

 rect method of registering the density and distinguish- 

 ing the difference between haze and fog is necessary ; 

 but fortunately there is a fair approximation to this 

 complete registration of London fogs published by the 

 Meteorological Office in their daily reports. The obser- 

 vations are made every morning at Brixton, and every 

 afternoon at Victoria Street, and from a paper by Mr. 

 Brodie, on " Some Remarkable Features in the Winter of 

 1890-91," published in the Journal of the Royal Meteoro- 



NO. I 149, VOL. 45] 



logical Society, I learn that the number of fogs thus 

 registered which have occurred each winter since 1870 

 is as follows, winter being represented by the months 

 December, January, and February. I have divided these 

 20 years into four groups of 5 years each : — 



Between 1870 and 1875, 93 fogs occurred. 

 ,, 1875 and 1S80, 119 ,, ,, 



,, 1880 and 1885, 131 ,, ,, 



,, 1885 and 1890, 156 ,, ,, 



It appears, then, that during the last twenty years there 

 has been a steady increase in the number of winter fogs. 

 I am not aware of any data to prove whether the density 

 of these fogs has increased, but it is probable that the 

 increase of number of fogs largely depesds upon an 

 increase of atmospheric impurity, and the conversion of 

 haze and mist into obvious fog ; and as the great colour- 

 ing matter of fogs arises from the combustion of coal, 

 I have drawn up the following table from information 

 which has been kindly furnished to me by Mr. G. 

 Livesey and Mr. J. B. Scott, of the Coal Exchange. It 

 gives the amount of coal really consumed annually in 

 London ; it does not include the coal used by the difter- 

 ent gas companies. For the first five years, the amount 

 given in the table is rather too high, as the quantity used 

 by the suburban gas companies could not be ascertained 

 and deducted. The quantities apply to what is known as 

 the London district — an area, on an average, of 15 miles 

 round London. The table shows an absolute increase, 

 during the last fifteen years, of 2,000,000 tons of coal — 

 that is, half as much again is now burnt as was burnt in 

 1875. 



Coal consumed in Loudon (that used by Gas Companies 

 deducted). 



Supposing only i per cent, of sulphur in this last yearly 

 amount is converted into sulphuric acid (H2SO4) and 

 passes into the air ; this would give 195,720 tons of this 

 acid. 



The five years' averages of winter fogs, we have seen, 

 give a steady increase, but obviously the number each 

 winter will vary much with the atmospheric conditions : 

 for instance, last winter was remarkably favourable for 

 the development of fog ; for, again taking the last twenty 

 years, the average number of days of fog during the 

 winter is 25, but last winter the actual number was 50. 



The general atmospheric conditions which induce fogs 

 are a still and moist air and a high barometer— a state ot 

 the air most usual under anticyclonic conditions. The 

 immediate determining cause, however, of a fog is usually 

 a sudden and considerable fall of temperature. Mr. 

 Brodie also points out that last winter was a time of 

 calms ; the percentage of such days on the average for 

 the last twenty years is 97, but last winter the number 

 was 22. Emphatically, he says, it was an anticyclonic 

 winter. 



A form of fog, well termed a " high fog," now frequently 

 occurs in London. The lights in a street during this form 

 of fog are often as visible as on clear nights, but above 

 hangs a fog so dense that the darkness of night may pre- 

 vail during the day. This particular form of fog appears 



