222 PHENOMENA CONNECTED WITH CLOUDY CONDENSATION. 



as tlie size of the water i-articles is increased. When the expansion 

 begins, blue is the first distinct color to appear, but very pale yellow 

 and slightly reddish colors have been noticed before the expansion was 

 sufticient to produce the blue. These reddish colors can be seen very 

 distinctly when we use an excessively great number of particles, and 

 they are best seen with gaslight. These reddish colors imperceptibly 

 change into blue as tlie expansion is increased, and the blue in turn 

 changes by minute degrees into green with further expansion, and the 

 green in turn changes to yellow; then a brownish color appears, which 

 changes to a somewhat mixed purple; then the blue returns again, to be 

 followed by green and yellow, as the expansion is still further increased. 

 It is not easy to get this sequence of colors carried so far. Sometimes 

 one stroke of the pump only carries the color on to yellow; sometimes 

 it may go to the second blue or green, but less frequently to the second 

 yellow. The final color depends on the number of particles present. 

 It is necessary to have a good many drops, so that the color may be 

 distinct, and yet not too many or the expansion may not be sufficient 

 to grow the particles large enough to give the second series of colors. 

 It is found that a high expansion, produced by two or more strokes of 

 the pump, does not give satisfactory results. 



We have seen that by increasing the number of dust particles the 

 depth of color was increased; it therefore seemed possible that these 

 color phenomena might be made visible in even a short column of air, 

 and that they might be shown by means of ordinary glass tlasks, The 

 following experiment was, therefore, arranged : A tiask, about 18 cen- 

 timeters diameter, was fitted with an India-rubber stopi)er, tlu'ough 

 which passed two tubes. One tube was connected with a metal vac- 

 uum receiver already described, the other had a stop-cock attached to 

 it. The stop-cock was connected with a long metal pipe which led to a 

 wide tube placed over a snuill flame. Air charged with the products 

 of combustion was drawn into the flask through this pipe; when suffi- 

 cient impure air was drawn into the flask, the stop-cock was closed; 

 when the air in the flask was now suddenly expanded, it looked as if it 

 had been tilled with a transparent blue gas. The color, when held 

 against a white cloud, was almost exactly the same as that of the blue 

 sky. The color in the flask faded rapidly, as in the experiments with 

 the tube. The particles being very closely packed in most of these 

 experiments the subsequent change is all the more rapid. 



Effect of temperature. — To observe the eflect of temperature on these 

 color iflienomena, another tube was prepared, with glass ends, and 

 jacketed, vso that the air in it might be heated or cooled to any desired 

 temperature. Theresult was very much what might have been expected ; 

 at the difl'erent temperatures all thecolorsmade their appearance in the 

 usual order, but there was a considerable difterem-e in the amount of ex- 

 pansion required to produce a given color with change of temperature. 



