1893.] on Fog, Clouds and Lightning. 99 



present in the air a number of material particles to serve as nuclei 

 around which the condensation could take place. All air, he says, 

 contains dust; by which term he does not mean such dust as is 

 rendered evident in this room by the light scattered along the track 

 of the beam issuing from the electric lantern, which consists of 

 comparatively gross lumps of matter, but particles of ultra-micro- 

 scopical dimensions, " more tenuous than the motes seen in a 

 sunbeam." It is upon such minute specks of matter that water 

 vapour is condensed. Anything that increased the number of dust 

 particles in the air increased the density of the condensation by 

 affording a greater number of nuclei. Air in which a flame had been 

 burnt he supposed to be very highly charged with finely-divided 

 matter, the products of combustion, and thus rendered extraordinarily 

 " active " in bringing about condensation. And that, according to 

 Coulier's view, is the reason why such a dense fog was formed when 

 air which had been contaminated by the spirit flame was admitted to 

 our globe. 



On the other hand, air, even burnt air, which has been filtered 

 through tightly packed cotton wool, is found to be perfectly inactive. 

 No cloud or mist will form in it, however highly it may be super- 

 saturated. Coulier explained this fact by supjDosing that the process 

 of filtration completely removed all dust particles from the air. 



On the table before you is a globe containing air which has been 

 thus treated, and which is kept saturated by a little water. When 

 this globe is connected with the exhausted receiver, no trace of any 

 mist is produced : the air remains perfectly clear. We will now 

 admit a little of the ordinary air from outside, and again cool it by 

 expansion. Quite a respectable cloud is thereupon formed in the 

 globe. 



The experiments of Coulier were repeated and confirmed by 

 Mascart. The latter also made one additional observation which 

 may very probably turn out to be of great importance. He found 

 that ozone, or rather, strongly ozonised air, was a very active mist 

 producer, and that unlike ordinary air, it was not deprived of its 

 activity by filtration. 



Four or five years later, ail the facts which had been noticed by 

 Coulier, and others of an allied nature, were independently discovered 

 by Mr. Aitken, who has devoted much time ancl study to them and 

 made them the foundation of an entirely new branch of meteorology. 



Later, perhaps, we may see reason to doubt whether all the 

 conclusions of Coulier and Aitken are quite accurate, especially as 

 regards the action of so-called products of combustion. 



What has been said so far applies equally to the generation of 

 clouds and of country fog, for a pure unadulterated fog, such as 

 occurs in rural districts, consists simply of a cloud resting upon the 

 surface of the earth. The fogs, however, which afflict many large 

 towns, and London in a marked degree, appear to possess a character 

 peculiar to themselves. They are distinguished by a well-known 



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