228 Messrs. Lodge and Clark on Dusty Air in the 



of a tube upon which the light is allowed to fall. But the 

 easiest plan is to use a substance which generates its own 

 smoke, such as phosphorus. A common stick of phosphorus 

 in ordinary air produces copious white fumes, which, being 

 heavy, descend, and when illuminated a white coat and well- 

 marked descending wdiite plane are seen. The white coat is 

 very thick, and hence probably it is that no portion of the 

 black coat or plane is to be seen. If the air in the box is 

 gradually dried, however, oxidation of the phosphorus so far 

 diminishes as to enable an observation to be made upon the 

 phosphorus in the particle-laden air before it again begins to 

 emit smoke. This reveals the phosphorus surrounded by the 

 dark coat and plane. But as the temperature rises the oxi- 

 dation proceeds more rapidly, and the black coat and plane 

 gradually become more obscure, as the smoke coming from 

 the surface of the solid enters them. 



7. Experiments on the Settling of Dust. 



Black paper-sheets and metal plates blackened with cam- 

 phor smoke have been arranged in various positions in a box 

 full of the w T hite smoke of magnesia. It is observed that 

 those which are warm receive barely any deposit of dust, not- 

 withstanding the large volume of air which has passed near 

 them. This is not due to the deposit being blown away. 



Of surfaces at the temperature of the air those seem to 

 collect most dust over which the air is the most stagnant ; 

 the dust has time to settle on flat level surfaces by common 

 gravitation unless the motion of the air over them is too 

 rapid. Cold surfaces collect a large quantity of smoke, and 

 become coated over with a thick white deposit, as if it were 

 attracted by the solid. Among cold surfaces rank all those 

 walls of the box which are not directly warmed by the beam 

 of light. If warm smoke is blown through a tube on to such 

 a surface it adheres very largely, giving a thick local deposit 

 opposite the tube. 



Impressions showing the dark plane can be obtained by 

 placing glass plates near warm solids in dust-laden air; and 

 we have copied such impressions of the dark coat and plane 

 on to sensitive paper by ordinary photographic printing *. 



8. Experiments in different Media. 

 (a) Gases. — By far the larger number of our experiments 



# Mr. John Aitken would also appear, from an abstract in ' Nature ' 

 (31st Jan.), to have obtained these records upon glass. It is singular how 

 closely many of his experiments run with ours. 



