1892.] connected with Cloudy Condensation. 435 



rally is. There seems to be something preventing the dust and the 

 vapour in our atmosphere acting under ordinary conditions in such a 

 way as to colour the sun blue or green. Perhaps it may be the tendency 

 the particles have to differentiation. This tendency, we have seen 

 from the experiments, rapidly destroys all colour effects, and from 

 this we might suppose it would be impossible that the colours, if 

 produced by water drops, could remain in nature visible for so long a 

 time as they did. But it must be remembered that the particles in 

 the experimental vessels are extremely close together, and the vapour 

 exchanges can therefore take place quickly. If, however, the drops 

 were widely separated, the exchanges would take place slowly. For 

 instance, if the drops in 1 cm. were separated so as to form a column 

 1 mile long, with a section of 1 sq. cm., we should have the same 

 amount of colour in 17 miles that we had in the 17 cm. of air in the 

 flask, and the particles would be so far apart that differentiation 

 would then take place extremely slowly. But further, if the supply 

 of dust and vapour were constantly kept up by the volcano, the 

 colour phenomena would continue for the same reason that they 

 continue in a steam jet, namely, by the drops being constantly re- 

 newed. 



A New Instrument for Testing the Amount of Bust in the Air. 



As this investigation progressed it became evident that these colour 

 phenomena placed in our hands an easy and simple way of estimating, 

 in a rough way, the number of dust particles in the atmosphere of 

 our rooms, which might be useful for sanitary purposes. An instru- 

 ment was therefore constructed to see how far the idea could be 

 practically carried out. This new instrument we intend to call a 

 Koniscope. In its present form this instrument consists of an air- 

 pump and a metal tube with glass ends, which we shall call the test- 

 tube. The capacity of the pump should be from half to three-quarters 

 the capacity of the test-tube. Near one end of the test-tube is a 

 passage by which it communicates with the air-pump, and near the 

 other end is attached a stopcock for admitting the air to be tested. 

 The test-tube and air-pump may be attached parallel to each other, 

 and held vertically when observing. If this arrangement be adopted, 

 a mirror must be attached to the lower end of the tube. In practice 

 it is found to be more convenient to omit the mirror, and observe 

 with the 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 

 found that any want of uniformity in the colour of the field produced 

 by the air heated on the sides of the tube can be greatly obviated by 

 lining the inside of the tube with a non-conducting substance, and 



