218 Messrs. Lodge and Clark on Dusty Air in the 



The appearances presented by the body were observed 

 either by the eye, through a hand lens, or through one of 

 Quincke's microscopes provided with a micrometer eye-piece, 

 indicated in fig. 8. 



General description of Black or Dust-free Coat and Plane* — 

 Before proceeding to a more detailed description of the ap- 

 pearances about to be described under varying conditions, 

 we will briefly state what occurs when a rod of electric-light 

 carbon, in ordinary air holding magnesic oxide in suspension, 

 is illuminated at right angles to its axis at a place near the 

 observed end by the converged beam of the electric light, and 

 is viewed in a line with its axis. A water-cell conveniently 

 intercepts a part of the radiation and renders the action 

 slower. A careful examination at the first instant of turning 

 on the light will usually reveal the dust-particles in close 

 proximity to the surface of the rod, but in the case of a 

 black solid they almost instantly leave it, and after a barely 

 appreciable interval of time the upward and rapid convec- 

 tion-currents commence. There is now a thin ring of per- 

 fectly dust-free air and of great definiteness surrounding the 

 carbon rod, a little thicker on the illuminated side, but 

 distinctly traceable all round. On the top of the rod this 

 ring is seen to stream upwards, the up-streaming portion 

 being at first broad, but rapidly narrowing into a black line 

 of very appreciable width, which is often traceable for a long 

 distance through the turbid air of the box. The black ring 

 surrounding the carbon cylinder is the dust-free coat, the 

 black line rising from the top of it is the dust-free plane; 

 the junction of the two the base of the plane. 



On the under surface of the rod the particles are seen 

 rushing up to the edge of the dark coat, not entering it, but 

 curling away to the right and to the left, as if this coat were 

 the real surface of the solid. Fig. la shows the rod in section 

 surrounded by its coat and the plane on the top; fig. 1 shows 

 also the paths followed by the dust-particles. It will be 

 observed that the dust-particles follow the air-stream lines, 

 moving with the air, not through it : any considerable motion 

 of particles across stream-lines would soon obliterate the dark 

 plane, whereas it is very persistent and is seen sometimes 

 coiling itself up into a spiral. Another point of much in- 

 terest is the distribution of the velocities of the dust-particles 

 at different distances from the surface of the solid on a 

 horizontal line perpendicalar to its surface. In the case of 

 carbon the velocities are all so great that individual particles 

 are not readily traced, but under favouiable circumstances it 

 may be noticed that the particles reach their maximum velo- 



