neighbourhood of strongly Illuminated Bodies. 227 



ployed a platinum wire, fig. 6, enclosed in the box fig. 5, and 

 ignited by a current from a battery. The appearances are 

 unchanged, and the air of the box rapidly clears ; the white 

 smoke being deposited upon the cold sides of the box. When 

 volatile particles, such as amnionic chloride, are used, a 

 thicker coat is observed at high temperatures than with 

 magnesic oxide. This is probably in consequence of volatili- 

 zation ; for at low temperatures, such as 100°, no such differ- 

 ence is noticeable. When a rod of camphor is examined 

 in air laden with magnesic-oxide particles, a dust-free coat 

 and plane may be distinguished. But as the camphor gets 

 gradually heated it volatilizes and makes the coat on the 

 side near the lamp far wider than elsewhere. Hence it would 

 appear that the volatilization of the camphor keeps back the 

 dust-particles beyond the normal distance. The camphor- 

 particles can be distinguished amidst the particles of MgO 

 on the boundary of the coat and plane near the light, on 

 account of their crystalline sparkling nature. 



What influence the size of the dust-particles exerts upon 

 the thickness of the coat has not yet been fully made out 

 experimentally. 



6. Experiments on Bodies which themselves give off Smoke. 



We early noticed that a distinction had to be drawn between 

 smoke given off from the warm body itself and smoke which 

 belonged to the air external to the body. The latter seemed 

 to be repelled from the body; the former seemed to come into 

 contact with its surface. 



If tobacco-smoke is blown on to an illuminated strip of 

 common window-glass, the smoke nearest to its surface hovers 

 tenaciously about long after the more distant smoke has dis- 

 appeared; but a visible dark coat is still found to separate 

 the dust from the surface. 



There are many ways of getting a body covered by a layer 

 of smoke. One way is to blow it on to its surface through a 

 capillary tube ; another way is to pass the copper rod through 

 the axis of a narrow glass tube fixed in a cork at the end 

 of the box, so that the rod projects into the experimental 

 glass box (fig. 5), and blow smoke through the tube. The 

 smoke rushes along the surface of the rod, forming a sort of 

 tube, but is gently carried away as the rod gets warmer, in 

 the manner described, showing itself all the time as a white 

 coat and plane inside the dark coat and plane which separate 

 and keep back the general dust in the air of the box. 



Another way is to let smoke ooze through holes in the sides 



Q2 



