374 THE AIR OF TOWNS. 



as to project into the open air; B, a glass tower, about 30 inches high 

 and 1J inches in diameter, open at the top, and drawn out into a fine jet 

 at the bottom. Two side tubes are fixed to the tower, one near the bot- 

 tom and the other on the opposite side near the top. The tower is filled 

 to within 1 inch of the upper sidepiece with glass beads, and into the 

 open top a tap funnel is inserted through a tightly fitting cork. The 

 lower side tube is attached to the horizontal tube; the upper one, by 

 means of wide india-rubber tubing, to a combined meter and aspirator, C. 

 This is an ordinary wet meter converted into an aspirator by attaching 

 toothed wheels to the revolving drum and driving the wheels by means 

 of a wire cord passing over a pulley and carrying a weight. A series 

 of dials register the volume to the one-hundredth of a cubic foot. The 

 method of conducting the experiment is as follows: About 250 cubic 

 centimeters of a solution of hydrogen peroxide in water, containing about 

 1 milligram of active oxygen in each cubic centimeter, is poured into 

 the tap funnel, from which it is allowed to drop onto the glass beads 

 at the rate of about one drop a second. The liquid passes down and 

 out at the lower end of the tube through the jet, and falls into a flask 

 placed below. A drop of liquid which permanently fills the jet seals 

 it effectually from the entrance of air from the interior of the room. 

 After running through, the liquid is poured back into the funnel. The 

 weight being wound up, the volume indicated on the dial is read off, 

 and the drum set in motion. With a column of beads of about 20 inches 

 and a weight of 20 pounds, 20 cubic feet can be aspirated in an hour. 

 Once started, the ax>paratus needs no further supervision until either 

 the weight has reached the ground or the solution of hydrogen peroxide 

 has run out of the funnel. The period required for this is readily deter- 

 mined, so that no time is lost in looking after the apparatus. 



Thus we see that carbonic acid and sulphurous acid, as we should 

 have anticipated, rapidly increase during fog, and, although I have no 

 determinations of soot to record, the fact that it increases also is suffi- 

 ciently evident. 



If we assume that dust particles are the cause of fog, then it follows 

 that the thickness of fog depends upon the number of these particles 

 and the fog must be denser in the town than in the country. Moreover, 

 each particle of water floating as fog becomes coated with a film of 

 sooty oil — of that oil which forms so large a constituent of soot. What 

 is the effect? Evaporation is retarded and the fog persists longer than 

 it would were these particles composed of pure water only. To illus- 

 trate this, I wish now to refer to an experiment, which has been pro- 

 ceeding since the beginning of the lecture. I then called your attention 

 to the fact that in each pan of this balance I had placed a large watch 

 glass containing water. Onto the surface of one watch glass of water 

 I had poured a drop of oil, which spread itself out into a film. You now 

 observe that this pan has descended, showing that evaporation has 

 proceeded at a greater rate in the other pan. 



