1893.] on Fog, Clouds and Lightning. 105 



but which quite baffles me.* Is it possible that the condensing 

 vapours may contain dissociated atoms ? 



To return to the electrical effect. There are only two kinds of 

 chemical change that I know of which could be brought about in air 

 by an electrical discharge. Either some of the oxygen might be con- 

 verted into ozone, or the oxygen and nitrogen of the air might be 

 caused to combine, forming nitric acid or some such compound. The 

 former of these would not account for the action of the air upon the 

 jet, because, as we have seen, ozone is inoperative ; the latter might. 

 But if the activity of the air is due to the presence in it of a com- 

 pound of oxygen and nitrogen, then it is clear that an electrical 

 discharge in either nitrogen or oxygen separately would fail to 

 render those gases active. 



I arranged a spark bottle, inside which an induction-coil discharge 

 could be made to take place ; two bent tubes were passed through 

 the cork, one reaching nearly to the bottom for the ingress of the gas 

 to be tested, the other, a shorter one, for its egress. The open end 

 of the egress tube was fixed near the steam jet, and first common air, 

 then oxygen and then nitrogen were successively forced through the 

 bottle while the coil discharge was going on. All produced dense 

 condensation, but I thought that oxygen appeared to be a little more 

 efficient than common air and nitrogen a little less. 



This last experiment points to a conclusion to which at present I 

 see no alternative. It is that the action on the jet of an electrical 

 discharge is due in some way or other to dissociated atoms of 

 oxygen and nitrogen. There is nothing else left to which it can be 

 due. 



So far as Robert Helmholtz's explanation coincides with this 

 conclusion I think it must be accepted as correct. As to the precise 

 manner in which he supposed the dissociated atoms to act upon the 

 jet, it is more difficult to agree with him. He thought that the 

 abnormal condensation was a consequence of the molecular shock 

 caused by the violent recombination of the dissociated atoms in the 

 supersaturated air of the jet, the action being analogous to that 

 which occurs when a supersaturated solution of sulphate of soda, for 

 example, is instantly crystallised by a mechanical shock. 



To me this hypothesis, ingenious as it is, seems to be more 

 fanciful than probable, but I can only hint very diffidently at an 

 alternative one. To many chemical processes the presence of water 

 is favourable or even essential. Is it possible that the recombination 

 of free atoms may be assisted by water ? And is it possible that 

 dissociated atoms in an atmosphere of aqueous vapour may obtain 

 the water needed for their union by condensing it from the vapour ? 



According to Helmholtz, flames and incandescent substances 

 generally cause dissociation of the molecules of oxygen and nitrogen 



* Two chemists of the highest eminence have been good enough to consider 

 the problem for me, but they are unable to throw any light upon it. 



