240 THE POPULAR SCIENCE MONTHLY. 



a closed case of one hundred cubic feet, if hermetically sealed at 

 a temperature of 30, with the barometer standing at thirty inches, 

 would have to resist the pressure equivalent to the addition of ten 

 cubic feet, when the temperature rose to 60, and the barometer fell 

 to twenty-nine inches. Have we not now discovered the reason 

 why dirt enters closed spaces ? What shall be the remedy ? 



Seeing, then, that air will find an entrance, and in the nature 

 of things must get in well, we must let it in, not at innumerable 

 uncovenanted small crevices, but at our own selected opening, 

 specially provided. Then we are in a position to strain off the 

 dust by providing the selected opening with a screen, which acts 

 as a filter. These, then, are the general principles on which we 

 must act. The rest is a question of detail. The details range 

 themselves under three heads : 1. What is the most effective, or 

 the most generally applicable filtering material ? 2. Given the 

 filtering material, what ought to be the proportion between the 

 area of the screened opening and the cubic contents of the case to 

 which it has to be fitted ? 3. What, in any particular instance, is 

 the best situation for the filter ? 



What is needed in our filtering material is that it shall readily 

 allow air to pass through, and shall also possess the quality of ar- 

 resting in its meshes fine particles of dust. For some purposes it 

 may suffice to use a coarse canvas, the threads of which are not 

 too closely twisted and have an abundance of fine fibers project- 

 ing from them, thereby reducing the small squares of the woven 

 texture to a still finer mesh. The material I have used most fre- 

 quently is " bunting," but it has disappointed me. When exam- 

 ined by the microscope many of the small squares of mesh are 

 seen to be deficient in delicate fibers standing out from the 

 threads, which would enhance the filtering power of the texture. 

 Lately I have tried other materials, domette, flannel, and cotton- 

 wool between layers of muslin, such as is used for dressing 

 wounds under the name of Gamgee tissue. Cotton-wool is prob- 

 ably the most perfect filter. Indeed, so perfect is it that in the 

 new science of bacteriology it is used as an effective means of ex- 

 cluding dust and germs from flasks in which experiments are to 

 be carried on. In order to put various textures to an exact com- 

 parative test, an experiment was tried. Having selected six quart 

 bottles with wide mouths, I tied over the mouth of each a piece of 

 the filtering tissue which I wished to test. The bottles are not 

 liable to crack, as wooden boxes are ; the only access for the inter- 

 change of air in the interior was through the filtering texture. I 

 thus had a means of testing the comparative value as strainers of 

 the various materials. Within the bottles were placed glass slides 

 on which any dust that was carried in might settle. The experi- 

 ments were begun on May 5, 1891, and the slides were taken out 



