1892.] 



connected ivith Cloudy Condensation. 



431 



was closed ; when the air in the flask was now suddenly expanded, it 

 looked as if it had been filled with a transparent blue gas. The colour, 

 when held against a white cloud, was almost exactly the same as that 

 of the blue sky. The colour 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 effect of temperature on these colour phenomena, 

 another tube was prepared, with glass ends, and jacketed, so that the 

 air in it might be heated or cooled to any desired temperature. The 

 result was very much what might have been expected : at the different 

 temperatures all the colours made their appearance in the usual order, 

 but there was a considerable difference in the amount of expansion 

 required to produce a given colour with change of temperature. 

 At a high temperature each of the colours appeared with a less ex- 

 pansion than when the temperature was low. In making these tests 

 the number 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 only green. At a temperature a little over 50°, 

 two strokes made it green, while 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. These differences are due to more 

 vapour being present and being condensed, with the same amount of 

 expansion, when the air is hot than when it is cold. It should be 

 stated that in all cases the air was saturated, the inside of the tube 

 being wet. 



The tube was also cooled down to 6° F., but no difference was 

 observed in the nature of the phenomena. The particles at that tem- 

 perature seemed to be still in the liquid form. 



Light Transmitted. 



The light transmitted directly through the cloudy condensation has 

 been examined by means of a small spectroscope. One of the tubes 

 was mounted vertically, and a mirror placed at the lower end; the 

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

 a small mirror was placed between the spectroscope and the glass end 

 of the tube. This small mirror covered half the field of the spectro- 

 scope, and reflected 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 



2 g 2 



