426 Mr. J. Aitken. On some Phenomena [Apr. 28, 



struction in front of the nozzle. When any of these, either separately 

 or combined, comes into action, the change is always in the same 

 direction, and if the colour was blue, it changes to yellow. 



It may be as well to note here that the yellows produced by most 

 dense forms of condensation are far from fine, and cannot be com- 

 pared with the blues. The yellows are not at all unlike the colours 

 occasionally seen through smoke, or in a thunder cloud. Though 

 this is the case with the dense condensation produced by most of the 

 causes, yet a very fine yellow is obtained when high-pressed steam is 

 used. 



It has been suggested that, because an electrified jet causes the 

 light transmitted through it to be coloured of a dark yellow-brown, 

 and as the colour seen in thunder clouds is similar, that, therefore, 

 the lurid colour of thunder clouds is due to the electrification. From 

 what is stated above, it will be seen that electricity is only one of a 

 number of influences which can change the condensation of the steam 

 jet and make the light transmitted through it of a yellow-brown 

 colour. Further, there is no evidence to show that electricity has 

 any influence of this nature on the form of condensation taking place 

 in clouds, and we are hardly entitled to expect it to have any such 

 influence, as the conditions under which the steam condenses in a jet 

 are very different from those under which condensation takes place 

 in clouds; and we have seen that electricity has no effect on the 

 nature of the condensation when it takes place in a mixture of hot 

 moist air and cold air. There is still another fact which points to the 

 same conclusion. If, in the steam jet, the proportions of dust, pres- 

 sure, &c, are such as to give an earlier stage than the blue, suppose 

 the transmitted light be green, then the electrification may not change 

 it to yellow, but may only make it blue. At present, it is therefore 

 very doubtful whether the electricity in a thunder cloud has anything 

 to do with its colour. 



Colours observed in Cloudy Condensation produced by Expansion. 



Though previous experiments had made me well acquainted with 

 certain colour phenomena, seen when cloudy condensation is produced 

 by the expansion of moist air in a receiver, yet I had never ob- 

 served any colours in the light transmitted directly through the 

 clouded air, such as are seen in the jet of steam when enclosed in a 

 tube. It seemed extremely probable that the reason for this would 

 be, that when the condensation is produced by expansion, the process 

 is slow, and the particles will, therefore, be too few to produce any 

 colour effects. In a steam jet, the expansion, cooling, and condensa- 

 tion take place very rapidly, and for that reason the number of water 

 particles formed is very great. An experiment was therefore 



