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



densation takes place more rapidly ; and it has been f onnd that, the 

 more rapidly the condensation is effected, the greater is the number 

 of particles formed. If the condensation take place slowly, a much 

 less number of nuclei are sufficient to relieve the supersaturation, as 

 there is time for the movements of the water molecules to take place ; 

 but if the rate of condensation be forced, then the tension of super- 

 saturation compels a great many more dust particles to become 

 centres of condensation. The result of this is, that with two samples 

 of the same mixture of air and steam, if one of them be condensed 

 slowly, the clouding is thin, while if the other be condensed quickly, 

 it is thick. This action will come into play in the steam issuing at 

 high pressure, when the steam is rapidly expanded, cooled, and then 

 mixed with cold air. 



The increased density produced by increase of pressure also takes 

 place somewhat suddenly, though not quite so suddenly as when the 

 density is produced by the other causes. The jet first gradually 

 thickens as the pressure rises, then a stage is arrived at when it 

 somewhat suddenly becomes dense. When this last stage is arrived 

 at, neither electrification nor the products of combustion cause any 

 increase in the density. The first thickening is probably the result 

 of the quickening of the condensation and increase in the number of 

 dust nuclei ; and the sudden increase in density is probably due to 

 the temperature falling low enough for the films of the drops to 

 have a repulsive action, sufficient to prevent them coalescing. 



5. Rough Nozzles and Obstructions in front of Jets. 



If we use a nozzle of irregular form, or having roughened edges, it 

 is found that it gives a dense condensation at a lower pressure than a 

 nozzle of circular section with smooth bore and thin even edges. 

 This is owing to the irregularities in the nozzle producing eddies in 

 the jet, and mixing a greater amount of air with the steam, so cool- 

 ing it more and supplying it with a greater number of nuclei. It, in 

 fact, acts in the same direction as increase of pressure, and aids 

 pressure in producing its results with a less velocity of steam. 



An obstruction in front of the jet acts in a similar manner, if we 

 have a jet of steam of such a pressure that at the temperature of the 

 air it gives only the ordinary form of condensation. If now we place 

 an obstruction in front of the jet so as to produce eddies, the con- 

 densation at once becomes dense. Wind has also a somewhat similar 

 effect. The reason of the increased density in these cases is the 

 same as for the jets issuing from irregular nozzles. They all assist 

 the pressure in intensifying the density of the condensation, by lower- 

 ing the temperature of the jet, increasing the number of nuclei, and 

 quickening the rate of condensation. 



