PHENOMENA CONNECTED WITH CLOUDY CONDENSATION. 223 



At a high temperature each of the colors ap^jeared with a less expausion 

 thau when the teiiiperature was low. lu making; these tests the num- 

 ber of dust particles in the air must be kept as constant as possible. 

 For this purpose windows and doors should be kept closed for some- 

 time before beginning, and the experiments should be repeated without 

 change of conditions. When the air was cooled to about 35°, it took 

 two strokes of the pump to develop a full blue, and three strokes made 

 it oidy green. At a temperature little over 50^, two strokes made it 

 green, wliile-if the air was heated to about 80°, two strokes sent it past 

 blue and green and on to yellow, and less than one stroke made it full 

 blue. The ditt'erences are due to more vapor being present and being 

 coiulensed, with the same amount of expansion, when the air is hot 

 than when it is cold. It sliould be stated that in all cases the air was 

 saturated, the inside of the tube being wet. 



The tube was also cooled down to (P F., but no difference was observed 

 in the nature of the phenomena. The particles at that temperature 

 seemed to be still in the liquid form. 



LUjht transmitted. — The light transmitted directly through the cloudy 

 condensation has been examined by means of a small spectroscope. 

 One of thetubes was mounted vertically, and amirror placed at the lower 

 end; the spectroscope was temporarily mounted over the upper end of 

 tlie tube; a small mirror was placed between the spectroscope and the 

 glass end of the tube. This small mirror covered halt the tield of the 

 spectroscope, and retlected light from the same source as that reflected 

 by the mirror at the lower end of the tube, so that one half of the field 

 gave the spectrum of the light, and the other half the light after i)assing 

 through the clouded condensation. The conditionsintheexperimentare 

 too fleeting for satisfactory observation. The only thing noticed was a 

 darkening of the Mhole spectrum, with a greater absor})tion at certain 

 points than at others. When the light was blue, in additiou to tlie gen- 

 eral reduction in brightness, the red end was more reduced than any 

 other part, and there was also a very marked shortening of the spectrum 

 at this end. When the color was yellow the reverse Avas the case. The 

 blue was almost entirely cut out, while the yellow was far the brightest 

 part of the Sliectruin, 



An examination has also been made of the diffraction colors as seen 

 in the halos surrounding bright lights. The most convenient way tried 

 of observing these colors was to use an ordinary glass flask of 18 cen- 

 timeter diameter, connected with the metal vacuum leceiver, as 

 already described. For the source of light, gas may be used, but a 

 better result is obtained with the light of the sky. In order to observe 

 these colors easily the window should be closed, all but a narrow ver- 

 tical strip; and it improves matters to have all surfaces on each side 

 of the opening painted black. When the air in the flask is expanded, 



