216 Messrs. Lodge and Clark on Dusty Air in the 



observation of an apparent purification of air by contact with 

 a solid, which if followed up might have led a good deal 

 further. It was not followed up, however, though the remark 

 is made that " it would seem as if this kind of contact was 

 sufficient to purify the air without the aid of centrifugal 

 force;" and the paper concludes with another test of the 

 centrifugal force hypothesis, concerning which, finally, " no 

 absolute conclusion can be drawn." 



In the autumn of 1883 our attention was called to the 

 matter by Lord Rayleigb/s article in 'Nature;' and being struck 

 with the apparent total collapse of all explanations hitherto 

 offered, we proceeded to repeat the experiments w T ith some 

 care. The first result of any importance which we obtained 

 was the fact that e dark plane hitherto observed to rise 

 from a warm body, or to sink from a cold one, is only a con- 

 tinuation of a dark or dust-free coat of uniform thickness and 

 sharp outline which completely invests the body; and we were 

 led to the conclusion that this coat is the most important part 

 of the phenomenon, because the up or down streaming-planes 

 seem only to be that portion of the coat which is being con- 

 tinuously wiped off by convection-currents, the coat on the 

 body being as continually renewed by some action not by us 

 then understood. This fact, together with a few other results 

 having reference mainly to the effect of electrifying the solid 

 bodv, was communicated in a letter to 'Nature ' (26th Julv, 

 1883, vol. xxviii. p. 297). 



Since then we have continued the observations. We shall 

 first describe generally the methods of experiment and the 

 phenomena observed. 



General Methods of Experiment, — In all cases the electric 

 light has been employed to illuminate the bodies under exa- 

 mination in dusty media. For the examination in air and 

 gases two principal forms of apparatus have been employed. 

 Fig. 5 shows the glass box which has been used for ordinary 

 air. The sides and one end are of glass, the other end is of 

 wood and perforated with a hole for a cork; the top is also of 

 wood, but provided with a wide slot, which can be closed by 

 a glass plate, so as to allow the substance under examination 

 to be illuminated from above, or by perforated wooden covers 

 through which tubes, wires, &c. could be introduced when it 

 was desired to test the electrical or other condition of the 

 body under examination. The box is closed at the bottom by 

 standing it upon a blotting-paper pad on one of Quincke's ad- 

 justable glass-plated supports. A fragment of magnesium wire 

 burnt beneath the box served to introduce air laden with mag- 

 nesie-oxide particles ; and by removing the cover from the top of 



