270 BACON. 



it is well known that air is ionized by bubbling through water, the 

 evaporation of water, and the formation of fresh-water surfaces being 

 accompanied by the liberation of ions. 



C. T. E. Wilson 5 has studied the action of ultra-violet light on gases 

 from the point of view of the effect produced by the light on the for- 

 mation of clouds. The following discussion of his work by J. J. Thom- 

 son 6 would seem to be pertinent : 



Wilson found that with very feeble ultra-violet light, clouds were produced 

 by expansion when this exceeded a certain definite amount, just as in the case 

 of a gas ionized by Rontgen rays, and that the amount of expansion required was 

 just the same for the ultra-violet light as for these rays; this at first sight would 

 make it appear as if the ultra-violet light ionized the gas. However, Wilson 

 found that the clouds produced by ultra-violet light differed from those produced 

 by Rontgen rays, inasmuch as the former were not affected by strong electric 

 fields, whereas the formation of the latter was almost entirely prevented by such 

 fields. If the clouds due to ultra-violet light had been due to the ionization of 

 the gas, the ions would have been removed by the field and the clouds stopped. 

 At the same time the coincidence between the expansions required for the forma- 

 tion of clouds under ultra-violet light and the conditions when ions are present 

 is so remarkable that we are very reluctant to believe that the nuclei are different 

 in the two eases ; it seems to me that an explanation in harmony with the facts 

 is that charged ions do form the nuclei of the drops formed by weak ultra-violet 

 light, but that these ions are produced during the expansion of the gas and are 

 not present when the gas is at rest. These ions might arise in the following 

 way: We have seen in Chapter VII that under the action of strong ultra-violet 

 light visible clouds are formed without expansion, these clouds probably being 

 due to the formation of hydrogen peroxide, which, mixing with the water, lowers 

 the vapor pressure; now when the light is very feeble it seems probable that 

 there still may be a formation of drops of water which, however, in consequence 

 of the very small amount of hydrogen peroxide produced by the feeble light, 

 never grow large enough to be visible. Thus we may regard the air exposed to 

 the ultra-violet light as full of exceedingly minute drops of water; when the 

 expansion takes place, the air rushes violently past the drops and we have a 

 condition which in many respects is analogous to the bubbling of gas through 

 water. However, when air bubbles through water, there is, as Lord Kelvin has 

 shown, negative electricity in the air and positive in the water. Thus, when 

 the air rushes past the water drops we should expect the air to contain negative 

 ions, the positive ions being on the drops; * * * the formation of a fresh 

 water surface is accompanied by the liberation of ions; when the cloud of small 

 drops is formed by the weak ultra-violet light there is a creation of a new surface 

 of water and the probability of the liberation of ions, the positive ions being 

 carried by small water drops would only move very slowly under an electric field 

 and so might not be cleared out by any field it is practicable to apply. 



Vincent 7 has experimented on the clouds formed by ultra-violet light without 

 expansion. He finds that some drops are charged positively, others negatively, 

 while others are without charge. He was not able to detect the presence of 

 hydrogen peroxide. 



*Phil. Trans. A (1899), 192, 403. 



" Conduction of Electricity through Gases. Cambridge, 2d ed. (1906), 255. 



7 Proo. Cambridge Phil. Soc. (1904), 12, 305. 



