1892.] connected ivit7i Cloudy Condensation. 425 



open at both ends too much air is drawn in, and the effect is much 

 the same as if no tube were used. The end of the tube has, therefore, 

 to be closed to a certain extent, to produce the colour phenomena. 

 But when high pressed steam is used, no check on the circulation 

 through the tube is necessary. The steam nozzle should be placed 

 outside the tube and a little to one side, so that the eye can be 

 brought into a line with the axis of the tube and a clear field of view 

 obtained while the jet plays into the open end of the tube. This is an 

 experiment which well repays the trouble of making it. When the 

 amount of steam, dust, and other conditions are properly propor- 

 tioned, the colours seen are very beautiful. "With ordinary condensa- 

 tion the colour varies from a fine green to lovely blues of different 

 depths. The pale blues equal any sky blue, while the deeper blues 

 are finer than the dark blues seen in the sky, as they have noue of the 

 cold hardness of the dark sky blues, but have a peculiar softness and 

 fulness of colour. 



Suppose now the tube is fitted up pointing to a clouded sky, or 

 other source of light, and that the steam jet, under slight pressure, is 

 blowing through it. If the exit end of the tube be open, we shall see 

 very little colour, and what is seen is only near the origin of the jet. 

 If now we partially close the end of the tube with the glass plate, to 

 prevent the jet drawing in so much air, we shall find that colour 

 begins to appear, and that when the plate is properly adjusted the 

 tube looks as if filled with a transparent coloured gas. The first 

 decided colour to appear is generally green, though I think I have 

 frequently seen a pale crimson before the green was visible. If the 

 circulation be checked still further, the colour will change to blue of 

 a greater or less depth according to the conditions. 



The above are the effects which may be looked for when the con- 

 densation of the jet is ordinary ; but suppose it be now caused to 

 change to the dense form, then the colour seen through the tube also 

 changes. If, when the jet is condensing in the ordinary way, and 

 the transmitted light is green, we cause the condensation to change 

 to dense, then the colour also changes and becomes deep blue, or, if 

 the ordinary condensation gave blue, the colour changes, when the 

 jet is dense, to a dark yellowish- brown. But between the blue and 

 the yellow there is always an intermediate stage when all colour dis- 

 appears and the light is simply very much darkened. The most 

 common effect of the change of the condensation from ordinary to 

 dense is for the transmitted light to change from blue to a yellowish 

 colour, and it does not matter how the change in the condensation is 

 effected; the colour always changes in the same way. We can, 

 therefore, cause the colour in the tube to change by electrifying the 

 jet, by a supply of cold air, by a supply of the products of combus- 

 tion, by increasing the pressure of the steam, and by placing an ob- 



