226 Messrs. Lodge and, Clark on Dusty Air in the 



plane wavy and unsteady, whilst the coat was so thin that 

 only with the aid of the hand-lens could it be traced with 

 certainty on the side of the carbon directly exposed to the 

 heat of the lamp. The plane, moreover, was incomplete at 

 its base — unilateral, a further indication that the coat was 

 probably wanting on the colder or shadow side of the carbon- 

 rod. Under a pressure of about 6 centim. the coat was 

 wider than in air at the barometric pressure. The depen- 

 dence of the thickness of the dust-free coat upon pressure 

 was well marked; and already some preliminary numerical 

 determinations have been made, which we hope will shortly 

 enable us to determine the law of the dependence of the 

 coat upon pressure with some degree of accuracy. The 

 following figures for air serve to show that the thickness of 

 the coat varies at different pressures. They were made with 

 the apparatus shown in fig. 8. 



Pressure of 



air, 



Thickness of coat, 



in centim. of 



Hg. 



in centim. 



75-9 





•013 



20-8 





•027 



10-9 





•054 



3-9 





•083 



The variation with pressure of course suggests ideas con- 

 nected with the free path of molecules at the surface of the 

 body ; but the thickness of the coat is far greater than that 

 ordinarily reckoned as the length of free path corresponding 

 to the pressure. 



The effects of electrification on the dust-free coats and the 

 examination of electrical actions in dusty air generally were 

 preliminarily noted in ' Nature,' July 1883, but the investiga- 

 tion of them is yet incomplete. 



5 . Experiments with different kinds of Smoke. 



We have endeavoured to ascertain whether any of the 

 phenomena obviously depend on a drying or on a combustion 

 of the smoke-particles. Bearing in mind the very wide coat 

 which surrounds an incandescent platinum wire, it seemed to 

 us, even in this case, improbable that dust-particles were often 

 burnt up by it, although we are well aware of the ease with 

 which finely divided matter will oxidize. Chiefly with this 

 object in view, we repeated an experiment adduced by Tyndall 

 to show that combustion was a vera causa of the dust-free 

 up-streaming air, but we used the incombustible and non- 

 volatile particles of MgO. For this purpose we have em- 



