Page:Popular Science Monthly Volume 40.djvu/271

Rh by the Rhône 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 thermometer. 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 circumstances 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 débris 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 all substances as a fog-producer in towns is burned sulphur. No less than three hundred and fifty tons of the products of the combustion 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