FORMATION OF SMALL CLEAR SPACES IN DUSTY AIR. 241 



foreign matter." Lord Rayleigh also offers as a suggestion that the particles 

 may be thrown out by the centrifugal force, as the mixture flows in curved 

 lines round the obstacle. 



In a letter to Nature of July 26, 1883, Dr Lodge gives an account of some 

 experiments he made on the dark plane and on dusty air. Dr Lodge says — 

 " We are now pretty well convinced that differences of temperature have 

 nothing to do with the real nature of the phenomenon ; we find that solid bodies 

 have sharply defined dust-free coats or films oj uniform thickness always surround- 

 ing them, and that these coats can be continually taken off them, and as con- 

 tinually renewed, by any current of air." Dr Lodge also describes a number 

 of interesting electrical experiments on the dust, and makes many very valuable 

 suggestions, but comes to no definite conclusion. He says — " Why .the air near 

 a solid is free from dust we are not prepared to say." 



From these quotations it will be seen that the whole matter is involved in 

 considerable obscurity; and as the subject already had considerable attractions 

 for me, I determined to undertake an investigation in this particular direction. 

 My experiments were begun in summer, but it was not till November that the 

 greater part of the work was done. 



I have considerable difficulty in determining how it will be best for me to 

 place the result of this investigation on record. As a rule, it is best to take 

 the reader over the road traversed by the investigator, as the probability is the 

 difficulties of the one will be the same as those of the other, and the results 

 generally unfold themselves best when treated in this way. In the present 

 occasion this method is not suitable. The subject, though apparently simple 

 enough, Avas found to be much more intricate and complicated than was 

 expected. The result was, many a false scent was followed only to be given 

 up, so that I would be taking the reader to my conclusions by a long, winding, 

 and uninteresting path. It will therefore be better for me simply to describe 

 the result of the investigation from my present point of view. 



Apparatus used. 



The apparatus used was all of the simplest and least expensive kind. The 

 dust-box in which the experiments were made was a cigar-box, the lid of 

 which was removed and a piece of glass put in its place. When in use the 

 box was placed on its end, with the glass to the front. A window was cut out 

 of the left side of the box, extending from close to the bottom to near the top, 

 and coining close to the front of the box. The box was then painted black 

 inside. Holes were cut in the back of the box, or wherever required for the 

 introduction of the different pieces of apparatus, which shall be afterwards 

 described. As a source of illumination, two gas jets, placed close to each 



