CHAPTER I. 



EFFICIENCY OF THE PLUG-COCK FOG CHAMBER. 



1. Introduction. In the last few years I have had occasion to use the 

 fog chamber extensively for the estimation of the number of colloidal* 

 nuclei and of ions in dust-free air under a great variety of conditions. 

 These data were computed from the angular diameter of the coronas 

 of cloudy condensation; and it is therefore necessary to reduce all 

 manipulations to the greatest simplicity and to precipitate the fog in a 

 capacious vessel, at least 18 inches long and 6 inches in diameter. To 

 obtain sufficiently rapid exhaustions it is thus advisable to employ a 

 large vacuum chamber, and the one used was about 5 feet high and i 

 foot in diameter. The two vessels were connected by 18 inches of brass 

 piping, the bore of which in successive experiments was increased as far 

 as 4 inches; but 2 -inch piping, provided with a 2. 5 -inch plug stopcock, 

 sufficed to produce all the measurable coronas as far as the large green- 

 blue-purple type, the largest of the useful coronas producible in a fog 

 chamber by any means whatever. Moreover, it is merely necessary to 

 open the stopcock as rapidly as possible by hand, using easily devised 

 annular oil troughs at top and bottom of the plug, both to eliminate 

 all possible ingress of room air and to reduce friction. Fog chambers 

 larger than the one measured were often used, and it is curious to note 

 that the efficiency of such chambers breaks down abruptly, while up 

 to this point different apparatus behaves nearly alike. The vacuum 

 chamber is put in connection with an air-pump, the fog chamber with a 

 well-packed filter by the aid of stopcocks. Water nuclei are precipitated 

 between exhaustions from the partially exhausted fog chamber. 



2. The variables. After reading the initial pressures of the fog and 

 vacuum chambers, it is expedient to open the stopcock quickly and 

 thereafter to close it at once before proceeding to the measurement 

 of the coronas. Eventually, i. e., when the temperature is the same in 

 both the fog and vacuum chambers, they must again be put in com- 

 munication and the pressures noted, if the details of the experiment 

 are to be computed. 



*See Smithsonian Contributions No. 1309, 1901; No. 1373, 1903; No. 1651, 1906; 

 Carnegie Institution of Washington Publications No. 40, 1906; No. 62, 1907. In place 

 of the term "colloidal nuclei," the term "vapor nuclei" will be used in preference in the 

 text below. These vapor nuclei of dust-free wet air are probably aggregates (physical 

 or chemical) of water molecules. 



I 



