THE AIR OF TOWNS. 369 



Lectuke 3. — Town Fog. 



Before discussing the nature and effects of town fog, we will begin, 

 as in the first lecture in the case of carbonic acid, by seeking for its 

 origin. 



Town fog is mist made white by Nature and painted any tint from 

 yellow to black by her children ; born of the air of particles of pure 

 and transparent water, it is contaminated by man with every imaginable 

 abomination. That is town fog. How does this mist arise? It is 

 water vapor or steam always present in the air in varying quantities, 

 which by a fall of temperature suddenly appears either as mist or rain, 

 snow, hail, or dew, according to the extent and rapidity of cooling and 

 the amount of water vapor present in the air at the time. The follow- 

 ing experiments will make this evident: 



A little ether is placed in this bright silvered cup (fig. 20); on 

 rapidly evaporating the ether by blowing air through it by means of a 

 hand bellows the temperature is lowered, and the bright surface soon 

 becomes dimmed with a deposit of moisture from the air. 



If, on the other hand, I bring a flame under the jet of steam (fig. 

 21), which is now visible through partial condensation in the form of 

 mist, i. e., fine water drops, the mist suddenly vanishes, for the warmer 

 air can now take up the water in its invisible form as vapor. When I 

 remove the flame the mist again appears. 



There is one interesting and curious fact about the formation of fine 

 particles of mist or the larger particles we call rain drops or dew — that 

 the starting point, the nucleus, of each of these particles of water is a 

 speck of dust, a speck so minute that it is generally invisible to the 

 naked eye. Without dust there is no mist or rain or dew. It is solid 

 matter which is the starting point for the deposition of moisture. 

 What would happen if air free from dust were saturated with moisture 

 and the temperature fell ? Water would be deposited, but only on solid 

 objects. It would deposit on the ground and on our buildings. It 

 would stream down the walls of our houses and soak the surface of the 

 earth. Every solid thing out of doors would be wet, but no mist would 

 appear and no rain would fall. 



Mist is the offspring of vapor and dust. What is the character and 

 quantity of this dust? We know that it exists. We know that it is 

 very plentiful in our houses. As far as we know, it exists everywhere; 

 but of course the quantity varies and varies enormously, as we shall 

 presently see. 



Here is a slide (fig. 22) which shows some of the things composing 

 the dust of a dwelling room highly magnified. 



We find in it particles of soot, crystals, fibers, vegetable cells, spores 



and pollen grains, starch grains and meteoric iron, the remains of insect 



life, and living germs. Of the character of this dust, I shall have more 



to say in my next lecture. Much of it is so fine that it is invisible under 



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