EXPERIMENTS WITH IONIZED AIK. 7 



might be located. When the dust contents are increased, the cusp of the opaque 

 I'egiou appi-oaches the abscissa, and lience the color loci will be successively more 

 crowded together. 



In the chart, figure 7, only a single air tube (d, fig. 3) was available. The air 

 was heated to about 40° by the circulating steam in the drum. This was then shut 

 ofl: and the temperature and pressui'e at which the coloi's disappeared noted on 

 cooling. The mercuiy thermometer is scai'cely sensitive enough for such observa- 

 tions, and the temperatui'es of the diagram are pi-obably too high. The observations 

 of figure 8 were obtained with the apparatus shown in figure 3. There is greater 

 certainty near the cusp, which is higher. The cai'dinal features of the phe- 

 nomenon are, however, the same in both charts. 



All colors are cleai'ed and become fainter on opening the bottom window of 

 the color tube. The opaque margin as a whole shifts to the left, indicating 

 the occurrence of lower temperatures. Thus at about 16° : 



Pressure of blue : Pressure of yellow : 



Open, ^2 cm. Open, 45 cm. 



Closed, 25 cm. Closed, above 100 cm. 



The jet is sensitive to about one inch above the hole. No effect is produced 

 by introducing dusty air two or more centimeters above it. When it is merely a 

 question of producing opaque condensation without regard to coloi-, a wide jet 

 supplied copiously with steam at a vanishing pressure excess, is useful. Thus a flat 

 lava tip is very sensitive under these conditions. 



5. Ionizers and '^ drnV producers. — The most convenient and generally efli- 

 cient "dust" pi'oducer which I have found is phosphorus at a temperature some- 

 where between 20° and 30°. Thin discs, P, of it may be enclosed between pieces 

 of wire gauze, about as large as a dime, and mounted on a stem, as in figure 9 a. 

 When not in use, this duster is as usual submei-ged in watei', to be thoroughly 

 dried by squeezing it between folds of bibulous paper pi-ior to the next expei'iment. 

 It lasts indefinitely, since it is only the glow visible in the dark which is effective, 

 while the noxious smoke which sometimes shows itself at higher temperatures is 

 without a condensation-producing tendency. A better source of dust particles is 

 made, as in figui-e 9 b, by inserting strips of wire gauze soldered together at points 

 a, about an inch or less apai't, and holding discs of phosphoi'us P, in the interven- 

 ing spaces, into a wide tube, AB. This is drawn to a finer end at C, where a stop- 

 cock may be placed. When not in use the strips ai'e withdrawn and dropped 

 into water. 



If the end A is placed near the mouth of the color tube (6', figure 1) and the 

 narrow end is open, sufficient air traverses the tube to produce a color depending 

 on the degree to which the end is open. It is far better, how^ever, to send a defi- 

 nite current through the tube from an aspirator controlled by a screw stop-cock 

 like those attached to oxygen cylinders. In such a case any desirable color of 

 cloudy condensation may be maintained in the tube for an indefinite length of 

 time, assuming that the steam jet is equally constant. No diffusion is observable 



