EXTERNAL AIR OF WASHINGTON. 185 



to be examined is transferred to the slide, and which is often 

 allowed to rest on the slide for a time, while the soiled contents 

 of the drop are subsiding. When the glass tubes used for this 

 purpose have been heated to incipient fusion, so that the sharp 

 edges of their orifices are rounded, no particles of glass appear in 

 the specimen. 



Entire shells of acarini have been not uncommon in the air. 

 Other insect detritus have been mostly scales of lepidoptera and 

 parts of flies and spiders. The pappus bristles of many of the 

 composites^ especially the late flowering asters, are often found in 

 the air ; a common form resembles miniature stems of equisetnm. 



The curious pollen of the pine, and the leaf hairs of various 

 plants are among the commonest objects in the air of the early 

 spring. Later in the season the place of these forms is taken by 

 other kinds of pollen — notably, that of several kinds of grass 

 have been recognised. 



EXPLANATION OF PLATE XIX. 



Fig. 1. — Apparatus separating organic matter from the air : — 



J., Glass cylinder, closed by rubber stopper, through which 

 pass jB, a Richardson's atomiser cut short, and 0, a glass tube 

 ground as a stopper, into the bottle D. Air, forced in by 

 the hand-pump, £*, atomises the water in A^ and passes out 

 by the tube 0, carrying a small portion of the atomised 

 water with it, which is stopped in the bottles D and D' . In 

 D the tubes are ground to fit as stoppers. D' is closed by a 

 perforated rubber stopper. ¥^ pipette and rubber tube for 

 washing. 



,, 2. — Contrivance for collecting dust from the air on a glass slide, 

 described on page 184. 



