OCTOBEE 17, 1902.] 



SCIENCE. 



635 



But the case is much more definite, as the fol- 

 lowing- paragraphs may indicate. 



6. In this place I may again call attention 

 to the fact that if retarded evaporation were 

 effective in giving the nucleus permanence, if 

 the observed dissipation of the nuclei of solu- 

 tions were in any way dependent on evapora- 

 tion, and not on the motion of nuclei, then 

 those nuclei which are produced by shaking 

 solutions of hygroscopic solutes like CaCly 

 H.SOj, etc., which can not wholly evaporate 

 their water, should be more stable than other 

 saline nuclei. The results show emphatically 

 that this is not the case. Nuclei generated 

 from hygroscopic bodies and their rate of 

 evanescence is not exceptional and is no 

 greater in the first case nor less in the second 

 than that of saline bodies in general. Hence 

 if the nucleus is necessarily a solution in case 

 of the former solutes (CaCl,, etc.), it is reason- 

 able to suppose it always to be. I have found 

 that the pressure decrement 5 p <; 2 cm. of 

 mercury (the limit is lower, but my apparatus 

 in its present form does not allow me to go 

 below this) is more than sufficient to precipi- 

 tate the nuclei produced by shaking. For 

 such small decrements the equation .29 

 Sp/p^dO/ (273 + ^), where d is the temperature 

 in degrees Centigrade, may be assumed. Now 

 the decrement of vapor pressure, S-, corre- 

 sponding to M, is at 20° about (S- = .115^'; 

 whence 57r=.25 cm. for the observed excessive 

 dp. In other words, the vapor depression of 

 a few millimeters is certainly much more than 

 is required to stop the evaporation of the 

 nucleus; so that if this depression is to be 

 due to the solute, the solution need not even 

 be very concentrated. For the case of H^SO^ 

 at 20°, the nucleus would hold more than 

 seventy-five per cent, of water in a saturated 

 atmosphere, and at lower temperatures much 

 greater dilution would suffice. 



7. Inasmuch, therefore, as the nucleus, from 

 my point of view, occurs under conditions of 

 potential growth from a few molecules of dry 

 solute to a relatively weak solution, as the air 

 becomes more and more saturated, this growth 

 and diminution must be a common occurrence 

 in nature. The persistent atmospheric nuclei 

 will be more dilute from the surface of the 



earth upward. The question then arises wheth- 

 er such growth or change of concentration is 

 accompanied by electric charge quite apart 

 from what is usually known as ionization 

 (demonstrable presence of non-saturated 

 chemical valencies) ; in other words, whether 

 any change of size of these excessively fine par- 

 ticles is reciprocally accompanied by surface 

 electrification. To be more specific : In an inves- 

 tigation published in 1892, Lenard showed that 

 in presence of air pure water is electropositive, 

 a circumstance which he attributes to a mere 

 Volta contact effect. It needs but a trace of 

 saline solute to reverse the potential. Solu- 

 tions in presence of air are electroneg-ative, 

 and more so as a rule, as the concentration 

 increases up to. a definite value (6.5 per cent, 

 in case of NaCl) for which the negative charge 

 is a maximum. After this as concentration 

 increases the potential gradually tends toward 

 zero (attained for a solution stronger than 

 about twenty-five per cent., in case of ISTaOl). 

 Removal of nuclei by condensation and sub- 

 sidence is then virtually a removal of nega- 

 tive electricity, provided the positive air 

 charge is not simultaneously removed. Here 

 then the possibility of a mechanism, in virtue 

 of which growth or increasing dilution is 

 associated with increased negative charge for 

 the nucleus, is actually at hand; but the diffi- 

 culty at present rests with the removal of the 

 air charge. 



8. Briefly then, the point which I wish to 

 make is that the occurrence of charge is inci- 

 dent and not causal to the existence of the 

 nucleus. Wiat conditions its persistence and 

 condensational activity is purely thermo- 

 dynamic. What conditions the efficiency of 

 electric transfer is a secondary property, open 

 to investigation though as yet but little under- 

 stood, and which even in the same nucleus is 

 present in very variable amount or may even 

 be quite absent. The phenomenon in its elec- 

 tric aspects depends, therefore, fundamentally 

 on the critical density at which evaporation 

 ceases. 



In the above paragraphs I have endeavored 

 to indicate how the current lines of argument 

 bearing more or less remotely on atmospheric 

 electricity at present stand; to point out that 



