206 PHENOMENA CONNECTED WITH CLOUDY CONDENSATION. 



It has generally been stated that the eftect of the electrifl cation is sud- 

 den and marked, that whenever the jet is electrified it at once becomes 

 very dense. This however is due to the manner in which the jet has 

 generally been electrified. Some degree of potential is necessary to pro- 

 duce 11 discharge from the j)oint, and whenever the i)otential is high 

 enough to cause this, it is sufficient to charge the drops high enough to 

 give rise to a very dense condensation. But if we make the discharg- 

 ing point extremely fine, or assist the discharge by means of a flame, 

 then we may begin with electricity of a very low ijotential, and the 

 increase in the density may be made to begin by almost imperceptible 

 degrees and to increase slowly to the dense form by gradually increas- 

 ing the potential. 



We shall for the present leave the question of the effect ox^ the ordi- 

 nary and the dense forms of condensation on the light transmitted 

 through them, as it will be better discussed after we have considered 

 all the ways in which the jet may be made dense, and we shall now 

 pass on to consider the second of those given in our list. 



2. An increase in tJie numhcr of dust nuclei. — It has been noticed 

 by iirevious observers that a flame brought near the jet tended to 

 make the condensation dense; but, in describing the exi)eriments, a 

 confusion has generally been made between the flame and the pro- 

 ducts of the combustion taking i3lace in the flame. So far as I have 

 been able to observe, flame has no ettect on the density of conden- 

 sation. Neither a luminous flame nor the flame of a Bunsen burner 

 has any eflect so long as the products of combustion are kept away from 

 the jet. But if the products are drawn into the jet, .they have a very 

 marked effect either in increasing or decreasing the density. If the 

 flame»is near and the gases are hot, they nuike the jet nearly invis- 

 ible, but if the gases are cooled or are not in great quantity, then they 

 make the jet as dense as if it were electrified. The simplest way of 

 studying this latter effect is to bring the products of combustion to 

 the jet by means of a metal tube 2 or 3 centimeters in diameter, and 

 about half a^ meter long. A small flame about half a centimeter 

 high, i)laced below the level of the jet, is used. One end of the tube is 

 kept over the flame while the other can be brought near the nozzle. 

 It will be found that Avhen brought into that position the jet will at 

 once be(;ome dense, and when it is removed it will return to its ordi- 

 nary condition, and become dense again with every return of the 

 impure gases. 



The increase in density in this case is due to the greater number of 

 dust particles in the gases olfering a greater number of nuclei for con- 

 densation, and the result is a great increase in the number of water 

 particles and consequent thickening of the condensation, a 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 com- 



