PHENOMENA CONNECTED WITH CLOUDY CONDENSATION. 207 



bnstiou 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 electritied 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 sui)})lying" 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 inde- 

 pendent motion, as they will be more guided by the gases than larger 

 drops; there wnll therefore be fewer collisions, and not the same 

 tendency to the diminution of numbers by the coalescence of a number 

 of drops into one. It may be because of the small number of the col-' 

 lisious wiien 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. 



o. Cold or loic temperatnre 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 ])oint of great im})ortance, 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 condensa- 

 tion 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 vapor condensed and to its rapid re-evaporation; 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 correspondingly faulty. 

 There is an influence at work in the condensing jet which, though due 

 to temperature, is of tar more importance than the effect of the tem- 

 perature on the amount of steam condensed. 



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

 I had opened the window of the room where the experiments were 

 being made, and when the fresh air came in the jet began to behave 

 Itself in a most uncertain way. At one moment it w^as quite steady 

 ordinary condensation, and the next it would conduct itself as if elec- 

 trically excited. Even after the window ^vas closed it continued to 



