576 Prof. 0. Barus on the Change of the 



transmission give the same colour by Newton's interferences, 

 as no better method is immediately available. The ordinates 

 are the volumes per minute o£ charged air (blue = 1 litre/min.) 

 producing the colour given by the abscissa in the tube. 

 (Cf. Bulletin U.S. Weather Bureau, No. 12, 1895.) 



If these series are examined individually as far as blue, 

 they will be found to lie on lines with but slight concavity 

 upward. Beyond the blue the upward trend is marked and 

 rapid. The regularity throughout is as good as may be 

 expected when colour criteria, sometimes faint, are made the 

 basis of measurement. The curvature suggested is probably 

 real : it is possible, however, that the emanating intensity of 

 phosphorus is being overtaxed in proportion as the air-current 

 velocities are increased. This would be particularly the case 

 near and beyond the opaque region, which lies on the left side 

 of the diagram. Curves a and b are referred to in the Table. 



It is interesting to compare with these results the data 

 obtained with the earlier form of tube in which the " dust " 

 comes in contact with a simple jet laterally from without. I 

 will make selections of mean values from a large number of 

 my earlier data as follows : — The numbers denote the litres 

 per minute of air saturated with phosphorus emanation, 

 needed to produce the colour in the steam-tube, if full blue 

 requires 1 litre/min. 



Faint crimson, *25 ; faint yellow, *35 ; green, *55 ; blue- 

 green, *60 ; light blue, *90 ; full blue, I'OO ; violet, 1*50 ; 

 opaque, 2*00. 



These data are necessarily irregular as they show the mean 

 positions of a large number of observations for each colour 

 under varying conditions. Compared with the present 

 experiments, however, they lie quite within a common group 

 of values*. Indeed the series of values when the jet is 

 " dusted" in a great variety of ways from without, the nuclei 

 being borne into the tube by a convection-current of air often 

 travelling many feet to reach the jet, and the results of the 

 present paper, where active air is at once introduced into the 

 jet from within, are indistinguishable provided the same 

 " dust " value be given to one of the colours (blue), and the 

 charged air be not too much exhausted of nuclei. This result 

 will appear more clearly in subsequent papers. Here I will 

 add an immediate application. 



3. Having in an earlier paper shown that the volume of 



* The old data are marked X in the chart. In their saturation they 

 correspond to the curve b. Results to the left of the violet must be left 

 in abeyance till I can construct a duster giving the necessary saturation 

 without explosion. 



