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By Beatrice Taylor, 



JOHN AiTKEN, of Edinburgh, seven years ago, proved experi- 

 mentally that the presence of dust was essential to the 

 formation of fog and cloud. More recently, it has been 

 shown that fog may be cleared by the discharge of electricity in a 

 fog-clouded atmosphere. This dispersion of fog by electricity is, 

 however, at present too expensive a process to be of practical 

 utility. 



In order that the reader may understand how Mr. Aitken 

 came to the conclusion that dust was a necessary factor in the 

 production of fog, I will give the experiments that forced him to 

 adopt this view. I will also investigate the case of town fogs, and 

 show that, like the caterpillars of Microgaster glomerata * (often 

 abused by market gardeners), town fogs are blessings in disguise. 



Here is the first experiment. Two receivers were connected 

 with a boiler. Into one the air of the room was allowed to pass ; 

 the other was filled with air filtered through cotton wool, a process 

 which completely frees it from dust. Steam from the boiler was 

 admitted to the receiver containing ordinary air, and immediately 

 a white fog appeared within the receiver. Next, steam was 

 admitted to the receiver containing filtered air. Not the slightest 

 cloudiness was produced, although this receiver was just as full of 

 water-vapour as the one first experimented with. Therefore, we 

 may draw this conclusion : that dusty air — ordinary air — gives a 

 white cloud of condensed vapour ; dustless air no fogging. 



The question at once suggests itself, Why does not water- 

 vapour condense in its visible form in air free from dust ? It has 

 been found that particles of water-vapour do not combine with 

 each other to form a cloud-particle, but that vapour must have a 

 solid or liquid body — a " free surface "— on which to condense. 

 Vapour in pure air consequently remains uncondensed and super- 



* The parasite on the larva of the White Cabbage Butterfly, a visitor that 

 should be welcomed by every gardener. 



