DUST AND FRESH AIR. 241 



on January 6, 1892, and most carefully photographed by Mr. 

 Lafayette, and made into lantern slides. 



The bottles were placed near a window in a room in the build- 

 ing of the Leeds Philosophical Society, i. e., quite in the center of 

 Leeds. The materials tested were : canvas ; bunting ; ordinary 

 flannel ; domette flannel, rough side in ; domette flannel, rough 

 side out ; cotton- wool, one inch thick. 



The results of the experiments show that as a consequence of 

 eight months' exposure, including a week of the worst fog I ever 

 knew in Leeds, three of the filtering tissues admitted a very ap- 

 preciable amount of dust, viz., coarse canvas the most, bunting 

 coming second, ordinary flannel admitting less than either. The 

 other three bottles were screened, one with thick domette rough 

 side in, one with domette rough side out, and one with cotton- 

 wool about an inch in thickness. The last three show hardly a 

 trace of dust. Curiously, the cotton-wool shows a trace more 

 than the domette flannel. The explanation of this I suspect to be 

 that the cotton-wool was not tied firmly enough round the neck 

 of the bottle, which had no rim, and that some air passed between 

 the bottle and the wool, instead of through the wool. 



Another experiment which I tried was to fit up a cupboard 

 with panels of double domette flannel. After the fog, to my sur- 

 prise, the inner screen had become more or less black, showing 

 that black particles had passed into the cupboard, but with this 

 remarkable difference : whereas the outer flannel was almost uni- 

 formly black from top to bottom, the inner flannel was divided 

 into four squares of different shades of blackness, corresponding 

 to four spaces between shelves. Of these four, the lowermost 

 was almost as black as the outside, and the uppermost was almost 

 clean. I just mention this as a fact which needs an explanation, 

 but without suggesting one. 



There is one error which I think has been committed in the 

 screens made for me, and it was pointed out by my friend Mr. 

 White, the architect, of Wimpole Street. The filtering material 

 is likely to act more effectively if left loose and not stretched 

 tight, as when tense the interstices are stretched and made larger, 

 and when out of sight it might be very loose, almost baggy, with 

 advantage. 



Hoping to get some hints as to the comparative value of the 

 various textures under trial, I placed specimens of each under the 

 microscope. It is obvious that both canvas and bunting are of 

 too open a texture, having numerous small holes unguarded by 

 delicate fibers. Judging by the microscope, one would conclude 

 that of woven textures, probably flannel, and still more, domette 

 flannel, are the best, and this judgment seems to be borne out by 

 the experiments with the bottles. 



VOL. XLI. 19 



