PHENOMENA CONNECTED WITH CLOUDY CONDENSATION. 221 



The conditions causing the different colors. — For studying the coudi- 

 tioDS which give rise to the different colors seen in these tubes, tlie air 

 pump and the small tube will be found to be the most suitable. Sup- 

 posing these to be fitted up, the following will show how the colors 

 change with the conditions: 



First, the effect of the degree of saturation of the air. If the air be 

 dry the colors are not good, and some degree of expansion requires to 

 be made before any effect whatever is produced on the air; and when 

 the colors do ai^pear, it is only in the center of the tube that they are 

 seen, the space all round next the walls being free from condensed par- 

 ticles. As the humidity is increased, this unclouded space near the 

 sides of the tube gradually diminislies. The colors are therefore best 

 studied when the air is saturated and the inside of the tube wet. When 

 this is done the colors extend to the wall and completely hll the tube. 



Second, the effect of the number of dust particles in the air. If we 

 use the ordinary outside air, the colors are very faint or invisible. Sui> 

 pose some slight color is visible, then it will be found that a very slight 

 expansion, say one-fifth of a stroke of the pump, will give a pale blue, 

 and if the exijansion be increased the color will change. If now we 

 use air from a room where gas is l)urning, and fill the tube with it, we 

 shall now, on expansion, get a much deeper blue, and it will be observed 

 that a greater expansion must now be made to get the best blue, and 

 before the color begins to change. If we alter the conditions still fur- 

 ther, and fill the tube Avith air in winch is mixed a good deal of the 

 products of combustion, we shall find that the condensation is now so 

 dense that we can scarcely see through the tube; but it will be noticed 

 that the color is a very deep blue, and that a full stroke of the ])ump 

 was necessary to produce this deep blue, but in this case no change of 

 color was produced with that large degree of expansion. 



These experiments show that, with few dust particles, a slight exi)an- 

 sion will produce the best blue, and that as the number of i)articles 

 increases, the amount of expansion necessary to produce the best blue 

 also requires to be increased, the depth of color increasing with ttie 

 increase in the number of dust particles. The explanation of the difier- 

 ences here is very simple. With few particles a very slight expansion 

 will deposit enough moisture to make the small number of drops of the 

 size sufficient to give the best blue color ; but, 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 color if the number of 

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

 produced, the color changes; and we shall now describe the successive 

 colors which appear as the degree of expansion is increased, that is, 



