436 



Mr. J. Aitken. On some Phenomena [Apr. 28, 



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-condnctor. 

 When the inside of the tube is lined with it, the field of colour is 

 fairly uniform, owing to the cooling of the sides by the evaporation 

 when the air enters and when expansion is made. 



For illumination, no doubt day light is the best when it can be 

 obtained, as gas light is so deficient in blue rays that colour 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 few tests with an instrument of this kind, and find 

 it very easily worked ; and for many practical purposes it is suffi- 

 ciently accurate. It cannot, of course, compare with the dust 

 counter for accuracy ; but, on the other hand, it is a much less expen- 

 sive instrument, and tests can be made far more easily with it, and 

 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 

 graduated 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 colour which appeared. This colour would indicate the number 

 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 colours to appear, and if a great many 

 particles are present, the one stroke will not give the whole of the 

 first series of colours, but may stop at the blue. If the temperature 

 of the air were always the same, this method might be adopted, but, 

 as we have seen, an allowance would be required to be made for tem- 

 perature, as with a high temperature the same degree of expansion 

 carries the colour further up the series. 



Another method of graduating might be to note the amount of 

 expansion necessary to give any particular colour, say to give the 

 best blue. With few particles a slight expansion gives the blue, 

 while with many particles a much greater expansion is necessary. 

 But here again the effect of temperature comes in. Temperature 

 observations would therefore require to be taken, and a correction 

 made which it might be difficult to carry out in practice. 



At present the best plan of graduating seems to be to note the 

 depth of the blue produced, regardless of the amount of expansion 

 required to give it, and use only this quantity as an index. With 

 few particles the colour is pale, and as the particles increase in num- 

 ber the colour increases in depth. Perhaps some addition might be 



