222 Messrs. Lodge and Clark on Dusty Air in the 



thermometer was still rising rapidly, and soon attained 35°. 

 The water-vessel, which had been used to moderate the violence 

 of the converging electric beam, was then removed, and before 

 long the thermometer was at 100° C, and was still rapidly 

 rising. The coat and plane were now thick and exceedingly 

 well-developed. In a second experiment, and examining 

 through a hand lens, the coat was visible at 23°*7, and at 26° 

 it was fairly thick. It must be remembered that the whole of 

 the bulb was not warmed by the beam, and these temperature- 

 readings are therefore only rough indications. 



Probably common wood-charcoal gives the thickest coat of 

 any substance we have examined ; and although so bad a con- 

 ductor of heat, the coat is not confined to the part of the solid 

 immediately exposed to the beam. The thermometer was 

 withdrawn from the experimental box, and the brass steam- 

 tube out of the water apparatus (fig. 10) was inserted in its 

 place ; its surface was platinized. When this tube was heated 

 by steam, the coat was rather thicker than when heated only 

 by the beam from the lamp. The thickness of the coat on the 

 steam-heated tube differs little from its thickness on the lamp- 

 black thermometer-bulb at 100°. 



The direct effect of temperature is ordinarily inseparable 

 from the secondary effect of the convection-currents, which 

 increase with the difference of temperature between the solid 

 and the* air. When the temperature of the solid is high, 

 the coat is thick, although the convection-currents may 

 be very rapid ; for the cause which gives rise to the con- 

 vection-currents also gives rise to the black coat : and it 

 appears that the increased temperature facilitates the formation 

 of the dark coat more than the increased velocity of the con- 

 vection-currents thins the coat. Air-currents can be produced 

 by blowing through a capillary tube on to the surface of the 

 rod under observation, and the effect of such a current is to 

 reduce the thickness of the coat ; in fact the coat may thus 

 be rendered so thin as to be imperceptible. The rod may 

 be replaced by a metal tube, closed at one end and pierced 

 laterally by a fine longitudinal slit or small hole in the exa- 

 mined region. An india-rubber tube attached to the other 

 end of the tube allows air to be blown into the coat or plane. 

 In this way the dark plane may be deflected in any direction, 

 and by a very strong blast the coat rendered imperceptible. 

 A body at the temperature of the air is probably destitute of 

 a visible coat. On black surfaces it is hard to establish this 

 point, as the light necessary for their examination warms them 

 so rapidly. The careful study of a glass plate, or bright 

 metal surface, however, reveals the fact that the coat is absent 



