STRUCTURE OF DUST-FREE AIR. 149 



109. Ordinary nuclei. The persistent nuclei of section 108 were pro- 

 duced by radiation in a medium of damp air, with the specific object of 

 avoiding the introduction of foreign matter into the fog chamber. It 

 is well known, however, that nuclei are producible by any profound 

 method of trituration which may be mechanical, as in the comminu- 

 tion of water by agitation or by the impact of jets. The resources 

 used may be of a more refined physical character like ignition or high 

 electrical potential, or of a chemical character like combustion and 

 the slow oxidation of phosphorus, etc. It is noteworthy that in all 

 these processes not only is ionization present, but that the ionization 

 and the nucleation produced in any definite process are proportional 

 quantities. This important result is demonstrable with phosphorus 

 nuclei by using the condenser and electrometer as usual for the ioni- 

 zation, and the steam jet for the nucleation ; or, with water nuclei, by 

 comminuting water by the aid of jets in the fog chamber, determining 

 the uucleation by the coronal method and the ionization by discharg- 

 ing the air laden with water nuclei through a tubular condenser. In 

 both these cases a definite amount of nucleation or ionization is pro- 

 ducible, and may be varied under control at pleasure. 



The slopes of the lines in the relation between the coulombs per 

 second passing radially in the tubular electrical condenser and the 

 liters per second of air saturated with phosphorus nuclei passing longi- 

 tudinally through the condenser into the steam tube, differ in differ- 

 ent experiments, whereas the colors of the field of the steam tube 

 referred to volumes of charged air per minute are in general agree- 

 ment ; i. e., whereas the nucleation is a fixed quantity, the number of 

 electrons per nucleus varies with the incidentals of the experiment. 

 Inasmuch as the ionization is subject to relatively very rapid decay 

 while the nucleation persists, a result of this kind is to be anticipated ; 

 but detailed investigations on the rates at which the ions and the 

 nuclei are severally produced in any given process, and their relations, 

 seem to me to be of great importance, and are now in progress at this 

 laboratory. 



110. Ordinary dust-free air an aggregate of nuclei. The steam jet * 

 shows that nuclei of small relative size, but, nevertheless, large as 

 compared with the molecules of air, must normally be present in dust- 

 free air ; for the axial colors may be kept permanent at any stage by 

 fixing the supersaturation. Such nuclei may be called colloidal 

 molecules, even the largest being much smaller than the ions. More- 

 over, the available nuclei to be reckoned in millions per cubic centi- 

 meter increase with enormous rapidity with the supersaturation in 



* Cf. Barus : Bulletin U. S. Weather Bureau, No. 12, 1893, Chapter III. 



