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Mr. J. Aifcken. On some Phenomena [Apr. 28, 



slight expansion will deposit enough moisture to make the small 

 number of drops of the size sufficient to give the best blue colour ; 

 bat, as the number of particles is increased, more moisture must be 

 deposited before the increased number of drops are made large enough 

 to give a full blue ; hence with a larger number of particles a greater 

 expansion is necessary to produce this effect. 



Third, the effect of the size of the condensed particles. As has 

 been stated, a slight expansion produces a blue colour if the number 

 of particles be small, and if the expansion be increased after the blue 

 is produced, the colour changes ; and we shall now describe the 

 successive colours which appear as the degree of expansion is in- 

 creased, that is, as the size of the water particles is increased. When 

 the expansion begins, blue is the first distinct colour to appear, but 

 very pale yellow and slightly reddish colours have been noticed 

 before the expansion was sufficient to produce the blue. These 

 reddish colours can be seen very distinctly when we use an ex- 

 cessively great number of particles, and they are best seen with 

 gas light. These reddish colours imperceptibly change into blue as 

 the 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 colour appears, which changes 

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

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

 It is not easy to get this sequence of colours carried so far. Some- 

 times one stroke of the pump only carries the colour on to yellow ; 

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

 to the second yellow. The final colour depends on the number of 

 particles present. It is necessary to have a good many drops, so that 

 the colour 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 colours. 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 colour was increased ; it therefore seemed possible that these 

 colour phenomena might be made visible in even a short column of 

 air, and that they might be shown by means of ordinary glass flasks. 

 The following experiment was, therefore, arranged : — A flask, about 

 18 cm. diameter, was fitted with an india-rubber stopper, through 

 which passed two tubes. One tube was connected with the metal 

 vacuum receiver already described, the other had a stopcock attached 

 to it. The stopcock was connected with a long metal pipe, which 

 led to a wide tube placed over a small flame. Air charged with the 

 products of combustion was drawn into the flask through this pipe ; 

 when sufficient impure air was drawn into the flask, the stopcock 



