2 VAPOR NUCLEI AND IONS. 



rated from the air by the filter and they require the smallest degree of 

 supersaturation of water vapor to precipitate condensation. They are 

 usually but not always (necessarily) foreign bodies in the air; at least 

 they are producible in dust-free air by the X-rays of sufficient intensity 

 and by other radiation. The second class comprises the fleeting nuclei. 

 They are often charged and then called ions. They persist for very 

 short periods of time, usually vanishing within a minute. They can 

 be maintained, therefore, only in the presence of radiation, corpuscular 

 or undulatory, from which their intimate association with electrification 

 or with ultra-violet light is manifest. Such radiation may occur spon- 

 taneously within the body of a gas during the state of generation. The 

 sizes of these nuclei are intermediate between the first or dust-like class 

 and the third class. This comprises the colloidal nuclei of dust-free air, 

 which are virtually persistent, inasmuch as they are a structural part of 

 the body of the gas and are reproduced as soon as removed. They require 

 the highest degrees of supersaturation for condensation and are without 

 electrification. 



All these groups may be made to pass continuously into each other. 



I shall use the term "nucleation" to denote the number of nuclei per 

 cubic centimeter observed in any experiment. It will be understood 

 that this means the efficient nucleation; for nuclei may be and usually 

 are present, which are not detected by the exhaustion. They are com- 

 puted in the present chapter from the angular diameter (^=5/30, nearly) 

 of the coronas, for a given supersaturation. To specify their number one 

 must come to a conclusion as to whether nuclei are removed more rapidly 

 by the exhaustion than they can be replaced by the molecular system, 

 or whether the reverse is the case. If fleeting nuclei and colloidal nuclei 

 are supposed to be instantly reproduced we shall call the number n. 

 None are then virtually removable by the sudden exhaustion. Otherwise 

 the nucleation (corrected for the volume expansion) is called N . In this 

 case the exhaustion is more rapid than the reproduction of nuclei. Both 

 n and N, as well as s, will usually be given in the earlier tables; for the 

 discrimination between n and N is not possible, and 5 is specially favor- 

 able to the small nucleation which are apt to be crowded out of the 

 diagrams for n and N . 



Returning to the subject of this section, I may state that the radium 

 used was a weak sample (10 mg., 10,000 X) hermetically sealed in a thin 

 aluminum tube. In an earlier paper I showed that whereas the nuclea- 

 tion produced decreased very rapidly with the distance of the energizer 

 from the outside of the cylindrical glass fog chamber, the number of 

 nuclei within was apparently the same throughout the length of the axis. 

 Pressure differences, however, were usually kept below the fog limit of 



