EXTERNAL AIR OF WASHINGTON. 183 



But it is with the Microscopic examination of the Air that we 

 feel most interested, the materials for which have been obtained 

 in the following different ways : — 



I. — Evaporation to dryness of a drop of the fluids resulting 

 from atomisation, and from condensation of moisture upon cold 

 surfaces. 



2. — Air-dust collected upon slides and small flattened watch- 

 glass-like discs, by simple exposure within and without doors. 



3. — Air-dust collected and retained by a drop of glycerine 

 upon glass slips exposed to the air. 



4. — Air-dust collected by the contrivance represented in Fig. 

 2, PI. XIX., which consists essentially of a double-winged vane, 

 turning freely upon an agate bearing, and carrying at the end 

 opposed to the wind a glass funnel with its stem bent at right 

 angles. Beneath the end of the funnel, held horizontally by spring 

 clips, is a glass slip, upon which is a drop of glycerine. A move- 

 able weight serves to balance the two ends of the vane, and the 

 upright rod {a) can be unscrewed from the tripod foot, and fixed 

 into any wooden surface. For this purpose it terminates in a 

 sharp steel point, not shown in the figure. 



The principle of this apparatus is not essentially different from 

 that of the aeroscopes devised by Pouchet, Madox, and Cunning- 

 ham. In these instruments the funnel has been drawn out to a 

 fine point, so as to impinge upon a thin cover or slip set vertically, 

 and the glycerine with which the latter was covered had a tendency 

 to absorb moisture from the air and run off from the glass after 

 long exposure. By bending the tube of the funnel at right angles 

 this difficulty has been obviated in the apparatus here figured, 

 and some improvement has been made in the details of its 

 construction. 



Moisture which had been impregnated with the contents of 

 the air, either by atomisation, condensation upon cold surfaces, 

 or by the natural washing process of rain and snow-falls, when 

 dried upon a glass slip has been found to contain the following 

 substances : — 



I. — Epithelium from skin and mucous membranes. 

 2. — Vegetable epithelium and unrecognised debris. 



