214 FOGS. 



as a condenser. Every blade of grass and every branch of a tree 

 would drip with moisture deposited by the passing air; our 

 dresses would become wet and dripping and umbrellas useless. 

 Worse than this, the inside of our houses would become wet ; the 

 walls and every object in the room would run with moisture. 



Before going further, I will state what conclusions we have 

 arrived at from the foregoing experiments : — 



I. — That whenever vapour condenses in the atmosphere, it 

 always does so on some solid nucleus. 



2, — That the dust-particles in the air form the nuclei on which 

 the vapour condenses. 



3. — That where there are many dust-particles a cloud appears 

 because each nucleus only gets a small allowance of vapour, and 

 is not made much larger or heavier, so that it continues to float. 



4. — That where there are few dust-nuclei, the amount of 

 vapour condensed on each particle increases, so that the size and 

 weight of the whole number of particles increases, producing in 

 them a tendency to settle down. Fogs, therefore, will only be 

 produced where dust-nuclei are abundant. 



5. — That if there were no dust there would be no fogs, no 

 mists, and probably no rain, while the supersaturated air would 

 convert everything into a condenser, on which it would deposit 

 its moisture. 



But it will be asked. Where does all this dust, so plentiful in 

 the air, come from ? And is it the dust we see in a sunbeam ? 

 Anticipating these questions, I will explain what the sources of 

 atmospheric dust are. Broadly speaking, anything that breaks up 

 matter into minute parts contributes a share to the general fund. 

 The spray of the ocean, when dried and converted into fine dust, 

 is an important source. Meteoric dust and volcanic dust form 

 probably another source of supply. But where populous districts 

 are concerned, combustion is certainly one of the most important 

 sources. Before, however, giving the experiment by which the 

 importance of combustion as a supplier of condensation nuclei 

 has been demonstrated by Mr. Aitken, it will be well to consider 

 what the fog-producing particles are. 



On entering a dark room into which the sun is shining through 

 a small opening in the shutters, the sun's rays have the form of a 



