224 Messrs. Lodge and Clark on Dusty Air in the 



3. Experiments made with different Substances. 



Various substances have been examined in dusty air, such 

 as copper, iron, zinc, carbon, glass, mica, selenite, vitreous 

 selenium, Iceland spar, potash, rock-salt, bismuth, blotting- 

 paper, black paper, white paper, chalk, polished silver, &c; 

 carbon moistened with various liquids ; water, ice. 



It may be stated, in a general way, that all these 

 substances, when examined in the beam of the electric 

 lamp, exhibit a remarkable similarity in their action towards 

 the dust in the air, although they no doubt exhibit dif- 

 ferences in the thicknesses of the coats. This is almost 

 certainly exclusively dependent upon the temperature to 

 which they are raised^ and it is very probable that a thin 

 layer of black camphor-smoke would make it difficult to 

 distinguish between them. Ordinary wood-charcoal yields a 

 coat remarkable for its thickness, in which respect it re- 

 sembles electric-light carbon. Black solids and bad con- 

 ductors favour the action by their high surface-temperature. 

 Repeated heating to redness and cooling in vacuo certainly 

 produces little, if any, diminution in the thickness of the coat 

 of electric-light carbon. 



Porous solids moistened with more or less volatile liquids 

 yield wide coats, and sometimes downward black planes. In 

 the dust-laden atmosphere they have been observed to give 

 rise to a partial double coat and two planes. When moistened 

 with two liquids of different boiling-points, no two distinct 

 planes and coats were obtainable, although the boundaries of 

 the observed coat and plane may not have been as distinct as 

 in the case of a solid in merely smoke-laden air. 



Liquid surfaces are capable of giving rise to dark planes of 

 great distinctness in the dusty air above them. This may be 

 well shown by attaching a thin horizontal platinum wire to 

 the ends of two copper wires passing through the tube of 

 a funnel, rendered water-tight by means of sealing-wax. 

 Water is then introduced into the funnel until it is about to 

 overflow. Above the surface of the water is the smoke- 

 laden atmosphere of the box. The platinum wire just beneath 

 the liquid surface is then warmed by means of a feeble current, 

 and the light turned on: a clear and distinct plane can then 

 be distinguished rising from the thin line of warmed liquid. 

 If the whole liquid surface be allowed to get warm a thick 

 dark coat is visible above it, and a large columnar dark 

 plane rises from the centre. 



Oar experience with mica was interesting. The first small 

 sheet which we tried was placed horizontally in the box with 



