EXPERIMENTS WITII IONIZED AIR. 27 



TABLE s— VARIATION OF COLOR WITH TEMPERATURE AT A GIVEN PRESSURE. 



'At 15° change of pressure up to 6 cm. produced indigo, purple, yellow, yellow-green. 



9. Corroborative Meiliod of Graduation. — I shall now describe a totally 

 different method of graduating the color tube, already referred to in connection 

 with figure 2 (§ 2). Here 6' is the color tube with its jet j, mirror vi, and influx 

 pipe 6". Instead of a single tube, two or more ionizers, P, P', P'\ are used simul- 

 taneously, each of them being charged with phosphorus and controlled by the inde- 

 pendent stop-cocks F, F', F". The air needed comes from the above gasometer 

 train, and after passing through the desiccator D, branches into thiee independent 

 currents. Obviously the air supply at D must be in excess. 



The method of using this apparatus is evident ; colors are compared by sum- 

 mation and difference. Thus if the colors for each tube separately as well as for 

 the tubes conjointly are known, and if one of the colors is defined in terms of the 

 dust content needed to produce it (Blue=l for instance), then as many equations 

 may be obtained as there ai'e colors. The method is thus very similar to the cor- 

 rection of a set of weights, etc. 



In table 6 I have inserted a number of such results, J9 and 0, being the steam 

 pressure and air temperature constant within the re(piired limits. I have also 

 added the data obtainable from the chart, figure 3, using the lower curve as this is 

 the moi'e definite. B denotes blue ; G, green ; V, violet ; O, opaque ; Y, yellow ; 

 Or, crimson. 



