PHENOMENA CONNECTED WITH CLOUDY CONDENSATION. 219 



very great. An experiment was, therefore, arranged in which the air 

 could be very rapidly exi)anded, so as to produce a high degree of 

 super- saturation, which it was ho])ed 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 i)ump, and 

 that the light be transmitted through some length of air. The plan 

 adopted was to use an air i)ump of ordinary dimensions, and for a 

 receiver a metal tube closed with glass ends. The first ai)paratus pre- 

 pared 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 centimeters diameter and 

 about half a meter long. It is provided with glass ends, fitted on air- 

 tight, and is provided with a bran(;h pipe at each end. One of these 

 branch pipes is connected with an air-pumj), and the other has a stop- 

 cock fixed to it. This stop-cock 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 phenomena are visible 

 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 centimeters 

 diameter and 19-3 centimeters 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 i)unip to fill the receiver 

 with air of the place, then close the stop-cock, and make a rai)id 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 i)articles, and re])eat the 

 experiment, very beautiful colors 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 colors, full, deep, and soft, in some 

 respects reminding one of iDolarization colors. 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 colors produced in this way are more 

 uniform and equal in all parts than those seen in the steam jet, unless 

 when the jet is very carefully adjusted ; the yellows are also much finer, 

 and the colors are more varied than those seen in the steam jet. 



There is however one most disappointing thing connected with these 

 colors produced by expansion ; they are very fleeting. Their full beauty 

 lasts but a second or two and they soon fade away, the color growing 

 dimmer and feebler every moment. This is owing to the difterentiatiou 

 which takes place in the particles forming the cloudy condensation. As 

 has been already explained, the small drops rapidly diminish in size 

 while the large ones increase, and as in these experiments the drops 



