ATMOSPHERE IN RELATION TO HUMAN LIFE AND HEALTH. 235 



The weight of the deposit was at Kew 30 grains in 20 yards. At 

 Chelsea the same area gave 40 grams, which is equivalent to 22 

 pounds to the acre, or 6 tons to the square mile. A large proportion of 

 the deposits of fog in smoky towns clearly arises from the imperfect 

 combustion of coal. On plants the deposit is sticky, like brown paint, 

 and is not washed off by water. A country fog is harmless in a green- 

 house; a town fog most destructive, killing soft-wooded plants, and 

 greatly damaging others. A very large number of plants will not 

 thrive in smoky towns. In Manchester, the deposit collected from 

 aucuba leaves gave 6 to 9 per cent of sulphuric, and 5 to 7 per cent of 

 hydrochloric acid, mostly in a state of combination. Three days' fog 

 deposited per square mile H hundredweights of sulphuric acid and 13 

 hundredweights of blacks. 



Among the results of smoky air in towns may be mentioned: The 

 discouragement of cleanliness and ventilation ; the constant deficiency 

 of light; the damage to plant life, so that only a few trees and plants 

 can live; the destruction and disfigurement of stone, cement, iron, 

 paint, wall papers, clothing, etc., and the depressing effect of dirt and 

 blackened streets on the people; losses to artists of all kinds who 

 depend on light; the lowered vitality of a large portion of the popula- 

 tion, and a contributory influence toward the rapid degeneration and 

 extinction of town families. 



In London the extra expenditure entailed is about £1 a head, or 

 more than the value of all the coal burnt in houses. The extra wash- 

 ing, painting, and repairs, and the loss of unburned carbon, etc., are 

 among the principal items in the account. 



The intensity of the ground fog depends largely on the amount of 

 cooling which the earth has previously undergone. At the beginning 

 of February, 1830, the ground in London was hard frozen with the 

 intense frost which had prevailed for some days. A moist southerly 

 current supervened and the temperature rose several degrees above 

 the freezing point. On the shady side of squares the fog then pro- 

 duced between the ground and 10 or 20 feet above it was so dense that 

 at 10 a. m. a lamp-post 4£ yards distant was invisible. In an ordinary 

 thick fog, such as that of January 11, 1888, objects are visible at thir- 

 teen times that distance. Above the shallow stratum of ground fog the 

 air was nearly clear and the smoke escaped. 



Such fogs are due partly to radiation into space, but also largely to 

 the mixture of the warm current with air which has become cold by 

 contact with the ground, and to radiation toward the ground. 



All radiation fogs disperse or greatly diminish when the sky becomes 

 clouded and reflects some of the warmth radiated from the ground. 

 They are not formed under a cloudy sky. 



2. Fog is frequently produced, sometimes on an enormous scale, cov- 

 ering an area exceeding that of the British Isles, by the mixture of 

 opposite currents of small velocity. The condition of atmosphere often 



