DAMP AND FOG. 191 



" villanous combination" of acrid matters, which 

 soil the people and their provisions, even while they 

 are in the act of eating. Broccoli, and also the 

 close-leaved vegetables, always have a nauseous 

 bitter taste in thick fogs. 



But the fog depends on the quantity of moisture 

 there is in the earth, or mud, or whatever happens 

 to be exposed to the air ; and so the density of the 

 fog must vary with that. Some parts of London 

 are on a thick bed of fine dry sand and gravel, which 

 allows the water to sink into the ground, so that it 

 is not there to cause fog. Others are on sludge or 

 mud, natural or artificial, and that works up bet ween 

 the stones of the pavement, forms mire on the sur- 

 face, and converts the street into a very successful 

 manufactory of fog ; and other parts again are on an 

 exceedingly tough clay, the surface of which is kept 

 cold by continual humidity and evaporation. 



We may here find a use in observing the effects 

 of the London fog ; for it will be found, where other 

 circumstances are the same, to be no bad indicator 

 of the healthiness of the different places. When the 

 air is more than usually humid, and the surfaces of 

 the walls in consequence cold, they melt dew out 

 of the warmer and humid air, just as the windows 

 of a room in which there are many people melt 

 dew out of the moist and warm air within ; or as 

 the surface of the earth and of vegetables melts dew 

 out of the warm air of the evening, which does not 

 cool so fast as these solid substances. The dew 

 of the fog takes the coat of the fog along with it ; 

 and thus, wherever the bricks and stones become 

 soonest discoloured, and the former show symptoms 

 of decay, and the latter get discoloured with green 

 mould, and other little plants, the place, whatever 

 may be its height above the mean level, is always the 

 most damp and unwholesome. Wherever the bricks 

 lose their colour fast, and become granular at the 

 edges, it will be found that the mortar is most de- 



