414 



Mr. J. Aitken. On some Phenomena [Apr. 28, 



result which, as has already been stated, the author proved some 

 years ago. 



The change in the appearance of the jet when the products of 

 combustion are brought to it, is exactly the same as that produced by 

 electrification. The whole jet becomes dense, the condensed particles 

 are visible nearly up to the nozzle, and the jet makes the same sound 

 as when electrified by silent discharge, and further, electricity of the 

 potential used does not make it any denser. 



It seems probable that the very great number of dust particles in 

 the products of combustion act in two ways : first, by supplying a 

 great number of nuclei ; and, second, as the number is greater the 

 drops will be smaller, and, on account of their small size, they will 

 have less independent motion, as they will be more guided by the 

 gases than larger drops ; there will, therefore, be fewer collisions, and 

 not the same tendency to the diminution of numbers by the coales- 

 cence of a number of drops into one. It may be because of the small 

 number of the collisions when the particles are small that electricity 

 has little or no effect on the jet when it is dense from a large supply 

 of nuclei. It is possible that some of the increased density produced 

 by the products of combustion may be due to the slight electrification 

 of gases from flames. But as the electrification from this source is 

 very slight, its effects will be extremely feeble indeed when the dust 

 particles are developed to the size of drops, so that the electricity 

 from this source is not likely to have much effect. 



3. Cold or Low Temperature of the Air. 



We now come to the third cause of the dense form of condensation, 

 namely, low temperature of the air. At first sight it may appear that 

 the above statement contains an already well-known fact. But while 

 in a certain sense this is so, yet there is one point of great import- 

 ance which, so far as I am aware, has not previously been observed. 

 If we were asked to state what is the effect of the temperature of the 

 air on condensation of the jet, we, probably, would say that when the 

 temperature of the air is high the condensation is very transparent, 

 owing to there being less vapour condensed and to its rapid re-evapo- 

 ration ; and that when the temperature became lower and lower the 

 jet gradually thickens as the temperature falls, owing to the greater 

 amount of condensation caused by the colder air. Such a description 

 is far from a full statement of the facts regarding the changes in 

 appearance with the fall in temperature, and the explanation is corre- 

 spondingly faulty. There is an influence at work in the condensing 

 jet, which, though due to temperature, is of far more importance than 

 the effect of the temperature on the amount of steam condensed. 

 When I first encountered this new influence it greatly puzzled me. 



