A CONTINUOUS RECORD OF ATMOSPHERIC NUCLEATION. 55 



saturation in the upper parts of the vessel, due to the precipitation of fog 

 particles with relatively slow diffusion of vapor, eventually becomes more and 

 more negligible. Hence the small region from which each nucleus draws its 

 liquid charge is apparently limited by the low rate of diffusion relatively to the 

 rate of subsidence of the nucleus. In large coronas, subsidence is slow, and the 

 region from which vapor reaches the nucleus is correspondingly large. In 

 small coronas, subsidence is rapid, and the region from which vapor is received 

 dwindles in local extent. 



7. Nuclei produced by the mixture of coal gas and air. Some time ago I 

 noticed that if coal gas is examined by the steam jet or color tube, as described 

 elsewhere, 1 a faint pink flush is seen in the field of the tube. This indicates the 

 presence of nuclei to the extent of many thousands per cubic centimeter in the 

 gas. Inasmuch as such nuclei could not be retained in the gas pipes (they 

 would soon be lost either by subsidence or diffusion), an explanation of the 

 phenomenon was difficult to suggest. Recently I examined the question by 

 the aid of the present method of coronas. Coal gas stored over water and" 

 suddenly cooled shows no condensation. It is therefore free from nuclei, as 

 would be anticipated. Filtered air under the same conditions behaves in the 

 same way. If, however, coal gas and filtered air are mixed and then examined, 

 nuclei are abundantly present, to the extent of several thousand per cubic 

 centimeter, showing that chemical reaction (attributable to the presence of 

 sulphide gas as an impurity) has taken place. 



If the air is introduced above the lighter coal gas, the nuclei are obtained 

 at once as a result of the mixture, by convection. If the coal gas is introduced 

 above the air, nuclei are not at first in evidence, but they appear later as the 

 result of diffusion at the surface of contact. The case of the steam tube is now 

 obvious, seeing that the gas is here necessarily introduced in contact with air. 

 These nuclei are not ionized, as special experiments with a condenser showed. 

 Very probably the product of the oxidation is sulphuric acid. 



1 See Experiments with Ionized Air , Smithsonian Contributions to Knowledge, No. 1309, 1901. 



