neighbourhood of strongly Illuminated Bodies. 223 



when first illuminated; its rapid formation may be seen. The 

 coat appears to reach its maximum thickness very rapidly. 

 At first we examined a body at the neutral temperature 

 as it passed gradually from a temperature lower than that of 

 the surrounding air to a higher. This method of examination 

 led one of us to suppose, and to imply in the letter to ' Nature ' 

 (26 July, 1883, vol. xxviii. p. 297), that the coat was present on 

 all bodies, whether warmer, colder, or at the same temperature 

 as the surrounding air. But this method of examination is 

 unreliable, as may be seen from the following. 



A rod of carbon is cooled to —21° C. in a test-tube sur- 

 rounded by ice and salt. The rod is quickly transferred to 

 the glass box (fig. 5), smoke introduced, and the beam, filtered 

 through water, turned on. The temperature being low, the first 

 thing seen is a bright down-stream of particles. Very soon 

 a dark streak in the middle of this appears, which dark streak 

 widens and becomes a sharp and well-defined down-streaming 

 dark plane, edged at first with the bright one, but ]ater be- 

 coming free from it. The convecaon-currents are still rapid, 

 and the only portion of a coat visible is the thickened base of 

 the (inverted) plane. This incipient coat is soon traceable 

 further. 



This incipient coat becomes visible further round the rod, 

 until the lower half is provided with a thin black coat* 

 Soon the downward convection-currents cease, the plane loses 

 its definiteness, and becomes confused with its thickening- 

 base and the coat itself, until the ill-defined mass of dust-free 

 air. turns around the rod of carbon, thus giving rise to a 

 hazy ill-defined adventitious coat, which, if removed, would 

 not be able to re-form at this temperature. The currents 

 then begin to ascend, and this adventitious coat gives place 

 to the true coat and sharp plane, already described as exist- 

 ing when carbon is w r arm. The appearance presented by the 

 cold but warming rod seems capable of some modification, 

 in consequence probably of variations in the rapidity with 

 which the carbon is heated, and of its being an imperfect 

 conductor. Thus occasionally the dark ill-defined dust-free 

 space which exists at the bottom of the rod, just as the pro- 

 cess of inversion commences, turns round towards the lamp, 

 and may be succeeded by the ordinary black coat and a plane 

 on that side, whilst, so far as has been ascertained, the coat 

 may be partially wanting on the side remote from the lamp. 

 Under these conditions one half of the base of the plane is 

 more or less absent. This constitutes the unilateral plane 

 shown in fig. 3. 



