THE MICRO-ORGANISMS PRESENT IN THE ATMOSPHERE. 
121 
Colonies, 121, 62, 64, 58 . Average = 76 colonies. 
Control-plate gave. 5 ,, 
106 litres of air yielded .'. 71 X 20 = 1420 ,, 
10 „ „ 1S4 „ 
III. Laboratory window (same). July 15, 1886. Wind, W. by S.W., varying to 
N.W., fairly strong. 
Volume of air aspirated = 92 litres. 
Construction of plug (single).—Glass-powder between two layers of glass-wool. 
Plug mixed with 20 c.c. of sterilised water, 1 c.c. of which gave on plate- 
cultivation :— 
Colonies, 37, 25, 53, 36. Average = 38 colonies. 
1 c.c. of sterilised water gave . . 6 ,, 
92 litres of air yielded .’. 32 X 20 = 640 ,, 
10 „ „ 70 „ 
IV. Laboratory window (same). July 16, 1886. Wind, W. by N.W., strong; 
dust blowing about in street below. 
No. 1. Morning. Volume of air aspirated = 106 litres. 
Construction of plug. —Loose. Powdered glass (y^-" thick) between two layers of 
glass-wool. 
Rate .—About 5 strokes # per minute. 
Plug mixed with 20 c.c. of sterilised water, 1 c.c. of which gave on plate- 
cultivation :— 
Colonies, 28, 21, 26 . . . Average = 25 colonies. 
Sterilised water only gave on control-plate 3 ,, 
106 litres of air yielded .'. 22 X 20 = 440 „ 
10 „ „ 42 „ 
Laboratory window (same). July 16, 1886. 
No. 2. Afternoon. Volume of air aspirated = 87 litres. 
Construction of plug. —Tight. Powdered glass (f" thick) between two layers of 
glass-wool. 
Plug mixed with 20 c.c. of sterilised water, 1 c.c. of which gave on plate- 
cultivation :— 
Colonies, 18, 25 . . . . Average = 21 - 5 colonies. 
The above control-plate applies equally 
to these plates. 
87 litres of air yielded .’. 18 - 5 X 20 = 370 ,, 
10 „ „ 43 
N.B.—The plugs in these two experiments were not further treated until the 
* Nine strokes of air-pump employed are equivalent to r L cubic foot = 2‘36 litres. 
MDCCCLXXXVII.-B. R 
