CHAPTER I. 

 NUCLEI OF PURE WATER. 



1. Introductory. In case of a fog chamber but 4 cm. high and 

 broad and lined with wet cloth, I was surprised to find that if each of a 

 succession of fogs precipitated on phosphorus nuclei is allowed to (appar- 

 ently) subside, i. e., to be completely dissipated without influx of air 

 before the next exhaustion is made, it nevertheless takes about 10 or 12 

 exhaustions before all the nuclei are precipitated. It follows, therefore, 

 that for very small fog particles the dissipation by evaporation in origin- 

 ally* saturated air is enormously more important than the dissipation 

 by subsidence. The fog particles do not, however, vanish completely, 

 but the evaporation terminates in persistent solutional water nuclei, 

 large enough to keep the air bluish or hazy. 



If the same experiment is made with rigorously dust-free air and vapor 

 nuclei, the fog particles vanish almost completely in a single slow evapor- 

 ation, so that but one additional exhaustion is needed to quite clear the 

 fog chamber. Very few water nuclei (several per cent) are left behind in 

 this case. The number increases when the evaporation is more and more 

 accelerated by the rapid influx of dust-free air. 



It is this feature of the experiment, the tendency of the water nuclei left 

 after precipitation of dust-free wet air on the vapor nuclei of its own 

 medium to persist in proportion as the evaporation is faster (more or less 

 compression or rise of temperature) , that is the chief burden of the present 

 paper. The fact that water nuclei may persist, while the enormously 

 larger fog particles from which they have been obtained all but evaporate, 

 is the interesting part of this result. Some restraining tendency! must 

 therefore be evoked by which the accelerating effect of convexity is much 

 more than canceled, for the nuclei here in question are produced in 

 rigorously dust-free air by the precipitation of water vapor on water 

 vapor. A solutional effect is therefore absent. 



To investigate the question it is expedient to begin with a medium of 

 phosphorus or solutional nuclei, to subject the fogs thereupon to more or 

 less rapid evaporation, and thereafter to compare these results with the 

 corresponding case for vapor nuclei. 



2. Rapid evaporation of fog particles. Phosphorus nuclei. In the 



following experiments I have endeavored to trace these results quantita- 



*Probably the rise of temperature is sufficiently rapid to render saturated air from 

 which a fog has been precipitated temporarily unsaturated. 



tThe earlier work has been quoted in Part I of Publication No. 96, Carnegie Institution 

 of Washington. 



I 



