372 THE AIR OF TOWNS. 



Number of dust particles in Leeds air. 



Per cubic inch. 



Wooclhouse Moor, northwest wind 530, 000 



Tennis Court, Yorkshire College 852, 000 



Town Hall Square, Leeds 1,228,000 



Paris Churchyard, Leeds 3,638,000 



Glasgow Town, northwest wind (Aitken) . .- 3, 736, 000 



Flour mill, Leeds 3, 113, 000 



There is one curious fact about these results, to which I would call 

 your attention. There are, you will observe, fewer dust particles in a 

 flour mill ? where the air is thick with dust, than iu the comparatively 

 clear air of the churchyard. This was a puzzle to me at first, but I think 

 it may be explained by the fact that the particles in the flour mill are 

 larger and therefore more visible. As to the size of the particles, they 

 may be accounted for possibly by coalescence produced by electrifica- 

 tion. This curious effect of coalescence of dust particles by electrification 

 may be easily demonstrated. We have only to connect one conductor 

 of an electric machine with a wire passing through the top of a bell jar 

 containing fumes from burning magnesium, when, on turning the 

 machine, a little whirlwind of particles is set up and in a moment, as 

 you see, all the solid matter has deposited in coarse grains round the 

 sides of the vessel (fig. 28). 



Let us now return from this digression to the fog once more and fol- 

 low its life history. 



With a calm atmosphere, a high barometer and a fall of temperature, a 

 film of water coats every little floating particle of dust, as it were, with 

 an overcoat to keep out the cold. A white fog slowly enshrouds the 

 town. Each particle of dust now heavily weighted with its unwonted 

 cloak of moisture has its progress impeded, hangs or falls, but does not 

 rise, and in its turn impedes the movement of the air. Stagnation of 

 the atmosphere is produced, especially as wind is light with fog. What 

 happens? An accumulation of products of combustion occurs, viz, of 

 carbonic acid, sulphurous acid, and soot, which under ordinary condi- 

 tions are rapidly dispersed. Our senses give us abundant evidence of 

 this in the case of soot and sulphurous acid. Our faces and clothes are 

 soon begrimed and our eyes and throats suffer from the irritating effects 

 of the acid. Carbonic acid shows a like increase, again illustrating 

 the well-known axiom that carbonic acid always comes in bad company. 



The table which I now project on the screen shows the increase of 

 carbonic acid during fog: 



Carbonic acid in London air. 1 



January 19, 1882. Slight white fog 0. 048 



January 25, 1882. Dense black fog 105 



February 1, 1882. Very fine 047 



February 3, 1882. Slight fog 062 



February 4. 1882. Dense black fog 107 



1 Dr. Russell, 



