IN THE PRESENCE OF DUST-FREE AIR AND OTHER GASES. 
285 
Observations were also made of the time taken by the drops to settle, as was 
gradually increased. This was of course very short when the condensation took the 
rain-like form. It showed a very sudden increase when the rain was replaced by fog. 
For example, such measurements were made in connection with the last observation 
given in the preceding table. 
Volv^. 
I'.S78 Colours scarcely visible, drops settled in a few seconds. 
I'381 Rings faint, drops took about one minute to settle. 
I'385 Rings brilliant, took several minutes to settle. 
All these methods of making evident the change from rain to fog agree in showing 
that this takes place when lies between 1'37 and 1’38. 
Colour Observations. 
The colour phenomena change so rapidly as ro/uj is increased from 1'38 to 1’40 
that consistent measurements were not possible. In the tables which follow the 
observations therefore begin where the brilliant greens previously referred to first 
appear. The colours are those seen on looking through the tube, almost, but not 
c|uite in the direction of the source of light. 
February 25. 
t = 13° C. 
t - 
March 3. 
18° to 19° C. 
Colour. 
i 
Colour. 
1-408 
Brilliant green 
1-410 
Green 
1-408 
1-410 
33 
I-4I2 
1-413 
35 
I-4I4 
1-416 
Blue-green 
I-4I4 
1-418 
Brilliant blue 
I-4I9 
Blue-green 
1-419 
Violet 
I-4I9 
Purple 
1-420 
33 
1-422 
1-420 
Reddish-purple 
1-424 
Brilliant red 
1-426 
Red 
1-426 
Red 
1-429 
Reddish-yellow 
1-428 
1-436 
Yellowish-white 
1-429 
1-448 
White 
1-4.34 
Reddish 
1-469 
Greenish-white 
1-437 
Reddish-white 
1-4.53 
Greenish-white 
1-458 
3 ? 33 
When the greenish-white fog appeared, the colour was the same from whatever 
