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



point to the conclusion that the dense form of condensation is due to 

 a large number of water drops, and the thinner form to a smaller 

 number, though of greater individual size. The only condition under 

 which it seems probable that the increase in number will not give 

 rise to increase in density is when the particles are so small that 

 they are unable to reflect waves of any colour of light. So far as has 

 yet been observed this never happens. However slight the amount 

 of expansion, the greater number of particles always gives the denser 

 form of condensation 



The action of the electricity on the jet does not appear to be any- 

 thing positive : it rather seems to prevent something which takes 

 place under ordinary conditions. For instance, electricity has no 

 effect in thickening the cloud of so-called steam rising from a hot 

 and wet surface. The electrically driven current of air from a point 

 when directed to the steaming surface has no effect whatever on the 

 density of the condensation. Nor has electricity any effect on the 

 steam rising from an open vessel. The small drops of water under 

 these conditions move but slowly, and there is but little tendency 

 for them to come into collision with each other ; there are, there- 

 fore, few collisions for the electricity to prevent, and little or no 

 thickening is produced by electrification under those conditions. 

 Further on we shall have frequent opportunities of seeing that the 

 dense form of condensation is the result of an increase in the number 

 of particles, and that whatever gives rise to an increase in the number 

 causes an increase in the density. 



When the jet is electrified and becomes dense, it has been noticed 

 by others that it emits at the same time a peculiar sound, and I find 

 that whenever the jet becomes dense, from whatever cause, it begins 

 " to speak." But when the density is due to electrification, the sound 

 is, however, slightly different from the sound emitted when dense 

 from any of the other causes. When dense from causes other than 

 electrification, the sound is similar to that produced by the jet 

 striking an obstruction ; but when electrified, the sound is a com- 

 bination of this sound with another due to the discharge of the 

 electricity ; and this second sound depends on the manner in which 

 the electric discharge takes place. If the discharging point is not 

 sharp, and the potential is just sufficient to cause discharge, then the 

 discharge is not continuous, but takes place at short intervals ; it 

 becomes, in fact, a series of disruptive discharges, and gives rise to a 

 fluttering noise. This fluttering sound is greatly increased if the 

 point terminates in a small ball of about 1 mm. diameter, and it is 

 entirely abolished if we use a very sharp point, or better, a flame. 

 The discharge with either the very sharp point or the flame is per- 

 fectly continuous, and nothing but the slight hissing that accompanies 

 all dense forms of condensation is heard when the jet is electrified. 



