2 54 THE POPULAR SCIENCE MONTHLY. 



be within the possibility of accurate enumeration. His plan is to 

 dilute a definite small quantity of common air with a fixed large 

 quantity of filtered, dustless air, and allow the mixture to be 

 supersaturated by water- vapor; the few particles of dust seize 

 the moisture, become visible in drops, fall on a divided plate, and 

 are there counted by means of a magnifying-glass. 



The instrument employed by Mr. Aitken has taken various 

 forms ; in fact, he has so far improved it that it can be carried in 

 the coat-pocket. But the original instrument, which we saw and 

 used, is most easily described without the aid of diagrams. But, 

 instead of his decimal system of measurements, we will use the 

 ordinary system, that the dimensions may be more easily grasped 

 by the general reader. Into a common glass flask of carafe-shape, 

 and flat-bottomed, of thirty cubic inches capacity, are passed two 

 small tubes, at the end of one of which is attached a square 

 silver table, one inch long. A little water having been inserted, 

 the flask is inverted, and the table is placed exactly one inch 

 from the inverted bottom, so that the contents of the air above 

 the table and below the bottom are one cubic inch. The observing 

 table has been divided into a hundred equal squares, and is highly 

 polished, with the burnishing all in one direction, so that during 

 the observations it appears dark, when the fine mist-particles, fall- 

 ing on it, glisten opal-like with the reflected light, in order that 

 they may be more easily counted. The tube to which the silver 

 table is attached is connected with two stop-cocks, one of which 

 can admit a small measured portion of the air to be examined. 

 The other tube in the flask is connected with an exhausting 

 syringe, of ten cubic inches capacity. Over the flask is placed a 

 covering colored black in the inside. In the top of this cover 

 is inserted a powerful magnifying-glass, through which the par- 

 ticles on the silver table can be easily seen and counted. A little 

 to the side of this magnifier is an opening in the cover, through 

 which light is concentrated on the silver table. This light, again, 

 has had to pass through a spherical globe of water, in order to 

 abstract the heat rays, which might vitiate the observations. 



To perform the experiment, the air in the flask is exhausted 

 by the syringe. The flask is then filled with pure filtered air. 

 One tenth of a cubic inch of the air to be examined is then intro- 

 duced into the flask, and mixed with the thirty cubic inches of 

 dustless air. After one stroke of the syringe this mixed air is 

 made to occupy an additional space of ten cubic inches ; and this 

 rarefying of the air so chills it that condensation of the water- 

 vapor takes place on the dust-particles. The observer, looking 

 through the magnifying-glass upon the silver table, sees the mist- 

 particles fall like an opal shower on the table, and counts the 

 number on a single square in two or three places, striking an 



