1893.] on Fog, Chads and Lightning. 103 



particles are certainly thrown off by electrical discharge, and the 

 idea that such particles promote condensation appears to be sup- 

 ported by the fact that if a piece of burning material, such as touch- 

 paper, is held near the jet so that the products of combustion can 

 pass into it, thick condensation is produced. [Experiment.] 



From a recent paper by Prof. Barus, published in the ' American 

 Meteorological Journal' for March, it appears that he also is of 

 opinion that such condensation is in all cases due to the action of 

 minute dust particles. Yet it is remarkable that Mr. Aitken, the 

 high priest and chief apostle of the philosophy of dust, gives no 

 countenance to the nucleus theory. He does not even advert to its 

 possibility. I imagine that his experiments have led him, as mine 

 have led me, to the conclusion that it is untenable. And this not 

 only in the case of electrical discharge, but also in the case of 

 burning matter. 



If we cause an electrical discharge to take place for some minutes 

 inside a suitably arranged glass bottle, and then, ten or fifteen seconds 

 after the discharge has ceased, blow the air from the bottle into the 

 steam jet, the condensation is not in any way affected. Yet the dust 

 could not have subsided in that time. And again, if we fill another 

 large bottle with dense clouds of smoke by holding a bundle of 

 burning touch-paper inside it, and almost immediately after the 

 touch-paper is withdrawn, force out the smoke-laden air, through a 

 nozzle, upon the jet — you can all see the black shadow of the smoke 

 upon the screen — nothing whatever happens to the jet. Yet a mere 

 scrap of the paper which is actually burning, though the ignited 

 portion may not be larger than a pin's head, at once darkens the jet. 

 Dead smoke (if I may use the term) exerts little or no influence by 

 itself: there must be incandescent matter behind it. The question 

 naturally arises, whether incandescent matter may not be sufficient 

 of itself, without any smoke at all. We can test this by making a 

 piece of platinum wire red hot and then holding it near the jet. It 

 is seen to be quite as effective as the burning touch-paper. Yet 

 here there can be no nuclei formed of products of combustion, for 

 there is no combustion ; there is simply ignition or incandescence. 



One other point I may mention. It is stated by Barus in the 

 paper above referred to that the fumes given off by a piece of phos- 

 phorus constitute a most efficient cause of dense condensation. This 

 is true if they come directly from a piece of phosphorus ; but if 

 phosphorus fumes are collected in a bottle and then directed upon 

 the jet, all traces of unoxidised phosphorus being first carefully 

 removed, they are found to be absolutely inoperative. Phosphorus 

 in air can hardly be said to be incandescent, though it is luminous 

 in the dark ; but it appears to act in the same manner as if its 

 temperature were high. 



All these facts seem to indicate that the several causes men- 

 tioned, electrical, chemical and thermal, confer upon the air in 

 which they act some temporary property — certainly not due to mere 



