PHENOMENA CONNECTED WITH CLOUDY CONDENSATION. 227 



observing;'. If this arrangement be adopted, ii mirror must be attached 

 to the lower end of the tube. In practice it is found to be more con- 

 venient to omit tlie mirror, and observe with tlie tube in any position, 

 simply directing- it to any suitable light. When this arrangement is 

 used, the pump, for convenience of working, should be attached at 

 right angles to the test-tube. It is fonnd tluit any want of unilormity 

 in the color of the field produced by the air heated on the sides of the 

 tube can l)e greatly obviated by lining the inside of the tube with a 

 non-conducting substance and keeping it wet. Blotting-paper is 

 found to do very well, as it holds plenty of water for saturating the 

 air and is a fair non-conductor. When the inside of the tube is lined 

 with it, the field of color is fairly uniform, owing to the cooling of the 

 sides by the evaporation when the air enters and when expansion is 

 made. 



For ilhimination no doubt daylight is the best when it can be 

 obtained, as gas-light is so deficient in blue rays that color is not well 

 observed. For convenience of observation, it is found to be best to 

 close the far end of the tube with ground glass, and when working 

 with artificial light ground glass must be used. 



I have made a i'ew tests with an instrument of this kind, and find it 

 very easily worked, and, for my practical i^urposes, it is sufficiently 

 accurate. It can not of course compare with the dust counter for 

 accuracy; but, on the other hand, it is a nuich less expensive instru- 

 ment, and tests can be made far more easily wuth it, aiul little special 

 knowledge is required. If we wish to get actual figures for the 

 amounts of dust indicated by this instrument, then it must be gradu- 

 ated by a dust counter. The indications at best, however, will only 

 be very rough approximations to the numbers. There are three ways 

 in which we might graduate this instrument. We might, for instance, 

 make one full stroke of the pump and note the color which appeared. 

 This color would indicate the nujuber of particles. For instance, if 

 there are few particles, one stroke will make the light first blue, then 

 green, then yellow, and then a second blue and green, and finishing 

 with yellow. But if there are a good many particles present, the same 

 amount of expansion will only make the first series of colors to appear; 

 and if a great many particles are present, the one stroke will not give 

 the whole of the first series of colors, but may stop at the blue. If the 

 temi)erature of the air were always the same this method might be 

 adopted; but, as we have seen, an allowance w^ould be required to be 

 made for temperature, as, with a high temperature, the same degree of 

 exi)ansion carries the color further up the series. 



Another method of graduating might be to note the amount of expan- 

 sion necessary to give any particular color, say, to give the best blue. 

 With few particles a slight expansion gives the blue, while with many 

 particles a much greater exi)ausion is necessary. But here again the 

 efiect of temperature comes in. Temperature observations would there- 



