DUST. 259 



by the Rhone spread out into the center of the Lake of Geneva, 

 the color assumes the deeper blue, rivaling in brilliancy any 

 water in the world. 



The phenomenon called a haze puzzled investigators until Mr. 

 Aitken explained it on the principle of the condensing power of 

 dust-particles Haze is only an arrested form of condensation of 

 water- vapor. If one half of a dusty pane of glass be cleaned in 

 cold weather, the clean part will remain undewed, while the dusty 

 part is damp to the eye and greasy to the touch. Why is this ? 



Fit up an open box with two pipes, one for taking in water 

 and the other for taking away the overflow. Inside fix a thermom- 

 eter. Cover the top edge of the box with India rubber, and fix 

 down with spring catches (so as to make the box water-tight) a 

 glass mirror, on which dust has been allowed to collect for some 

 time. Clean the dust carefully off one half of the mirror, so that 

 one half of the glass covering the box is clean and the other half 

 dusty. Pour cold water through the pipe into the box, so as to 

 lower the temperature of the mirror, and carefully observe when 

 condensation begins on each of the halves, taking a note of the 

 temperature. It will be found that the condensation of the water- 

 vapor appears on the dust-particles before coming down to the 

 natural dew-point temperature of the clean glass. The difference 

 between the two temperatures indicates the temperature above 

 the dew-point at which the dust condenses the water- vapor. Mr. 

 Aitken found that the condensing power of the dust in the air of 

 a smoking-room varied from 4° to 8° Fahr. above the dew-point, 

 whenever that of the outer air varied from 3° to 5-^°. 



Moisture is, therefore, deposited on the dust-particles of the 

 air which is not saturated, and condensation takes place while 

 the air is comparatively dry. before the temperature is lowered to 

 the dew-point. The clearest air, then, has some haze ; and, as the 

 humidity increases, the thickness of the air increases. In all haze 

 the temperature is above the dew-point. And in all circum- 

 stances the haze can be accounted for by the condensing power of 

 the dust-particles in the atmosphere at a higher temperature 

 than that required for the formation of fogs, or mists, or rain. 



But whence comes the dust ? Meteoric waste and volcanic 

 debris have already been mentioned. On or near the sea the air 

 is impregnated by the fine brine-dust lashed by the waves and 

 broken upon the rocks and vessel-sides. But the most active of 

 iall substances as a fog-producer in towns is burned sulphur. No 

 less than three hundred and fifty tons of the products of the com- 

 bustion of sulphur from the coal are thrown into the atmosphere 

 of London every winter day. But the powerful deodorizing and 

 antiseptic properties of the sulphur assist in sanitation ; and it is 

 better to bear the inconvenience of fogs than be subjected to the 



