220 PHENOMENA CONNECTED WITH CLOUDY CONDENSATION. 



are very close to each other these changes take place the more rapidly. 

 These changes are also taking i)Iace in the steam jet, bnt owing to the 

 constant supply of new drops the ohler ones are swept away before the 

 change is observed. The following experiment will however show that 

 these changes are taldng place in the steam Jet also. If, while the jet 

 is condensing and the transmitted light is yellow, we imprison some of 

 the jet by closing both ends of the tube, we shall hnd in an extremely 

 short time that the color will change to bhie, after wliich it will fade, 

 as the drops increase still further in size, and fall. In this experiment 

 we have a proof of the statement that when the jet is electrified the 

 drops are smaller than when not electrified, and not larger, as has been 

 supposed. As this experiment shows, if we begin with drops transmit- 

 ting yellow light, as the drops diminish in numbers and increase in size 

 the transmitted light changes to blue. 



The conditions of the experiments for i)rodnciug color by cloudy 

 condensation, produced by expansion, have been varied in a number 

 of ways. After ihe air has been cooled by the expansion, the layer of 

 cloudy air in contact with the walls of the tube rapidly acquires heat 

 from the metal, and the rise in temperature quickly evaporates the 

 cloudy particles and causes a clear space all round next the walls, so 

 limiting the color to the center of the tube. The receiver was there- 

 fore increased in diameter to get rid of the disturbing effects of the heat- 

 ing of the air on the walls of the tube, so as to have a larger mass of air 

 beyond this influence; but no decided advantage has been obtained. 

 It was afterwards found that the difficulty of studying these color 

 eft'ects in small tubes can be easily overcome by wetting the inside of 

 the tube. With this precaution the air next the walls is kept saturated 

 and the temperature of the wall is lowered by the heat given off to 

 evaporate the water, with the result that the color is the same all over 

 the field and close up to the walls. 



Large tubes might be used for showing these color phenomena to an 

 audience, a parallel beam of light being sent through them, which 

 would become colored when the dusty air iu them was expanded. One 

 large tube tried has a diameter of 7 centimeters and is 50 centimeters 

 long. With a receiver of that capacity it would behoj)eless to attempt 

 to i)roduce any color effects witli an ordinary air-pump alone; a vacuum 

 receiver has, therefore, been added to the apparatus. This receiver is 

 made of metal; it is lo centimeters diameter and 60 centimeters long, 

 with round ends. There are two tubes attached to it, one for connect- 

 ing it with the air-pump, and the other is provided with a. stop-cock, to 

 wliich a tube is attached, by which it is connected with the experi- 

 mental receiver. The stop-cock is closed, and the pump worked till 

 most of the air is taken out of the receiver; and when it is desired to 

 expand the air in the experimental tube the stop-cock is opened, when 

 a violent rush of air takes place, and the pressure is rai)idly lowered in 

 the experimental receiver and a dense color-producing form of cloud- 

 ing is obtained. 



