1892.] 



connected with Cloudy Condensation, 



427 



arranged, in which the air conld be very rapidly expanded, so as to 

 produce a high degree of supersaturation, which it was hoped would 

 cause a great number of dust nuclei to become active. To test this 

 idea, all that was necessary was that the receiver used for holding the 

 moist air should be much smaller than usual in comparison with the 

 capacity of the pump, and that the light be transmitted through some 

 length of air. The plan adopted was to use an air-pump of ordinary 

 dimensions, and for a receiver, a metal tube closed with glass ends. 

 The first apparatus prepared for this experiment was found to give 

 satisfactory results, and the alterations since made have not been of 

 any great advantage. 



The apparatus consists of a brass tube 2*3 cm. diameter and about 

 half a metre long. It is provided with glass ends, fitted on air-tight, 

 and is provided with a branch pipe at each end. One of these branch 

 pipes is connected with an air-pump, and the other has a stopcock 

 fixed to it. This stopcock is connected with a pipe for bringing to 

 the tube the air to be experimented with. If the tube be mounted 

 horizontally, the particles rapidly fall and the phenomenon is visible 

 for only a short time. The tube is, therefore, best mounted vertically, 

 and with a mirror placed at the lower end of the tube to reflect the 

 sky or other source of light up through the tube to the eye of the 

 observer. 



The air-pump used is a single cylinder instrument of 3" 17 cm. 

 diameter and 19'3 cm. stroke, so that its capacity is about three- 

 quarters that of the tube receiver. If we take the instrument outside the 

 house and make one or two strokes of the pump to fill the receiver with 

 air of the place, then close the stopcock, and make a rapid stroke with 

 the pump, little effect is produced on the light transmitted through 

 the tube. But if we take the instrument into a room where gas has 

 been burning, so that the air is full of dust particles, and repeat the 

 experiment, very beautiful colours are seen on looking through the 

 tube when the air is expanded. Or, better still, if we collect the gases 

 rising from a small flame and draw them into the tube, the result is 

 a display of an exceedingly lovely series of colours, full, deep, and 

 soft, in some respects reminding one of polarisation colours. As in 

 the steam jet, the blues are the finest, and the tube looks, at times, as 

 if filled with a solution of Prussian blue. The colours, produced in 

 this way are more uniform and equal in all parts than those seen in 

 the steam jet, unless when tire jet is very carefully adjusted ; the 

 yellows are also much finer, and the colours more varied than those 

 seen in the steam jet. 



There is, however, one most disappointing thing connected with 

 these colours produced by expansion : they are very fleeting. Their 

 full beauty lasts but a second or two, and they soon fade away, the 

 colour growing dimmer and feebler every moment. This is owing to 



