70 CONDENSATION OF VAPOR AS INDUCED BY NUCLEI AND IONS. 



adiabatic drop from 76 cm., 19.8 cm.), but it hugs the axis until about 

 0.33, after which it sweeps upward far beyond the chart into the hun- 

 dred-thousands. The position of Wilson's negative ions and positive 

 ions is indicated at 0.27 and above 0.31. Wilson's fog point would lie 

 at 0.36 in the chart and there would be an air curve to the right beyond. 

 Series III to X are taken from my earlier report (Carnegie Institution 

 of Washington Publication No. 62, 1907, p. 67). The serial number is 

 marked on the curve. 



41. Weak radiation. If a weak ionizer (radium io,oooX, 100 mg., 

 sealed in an aluminum tube) is placed at D = 40 cm. from the glass fog 

 chamber, the air curve rises slightly above dp/p = o.2$, becomes nearly 

 constant slightly above 0.27 until above 0.35, after which it also begins to 

 sweep with great rapidity into the hundred-thousands of nuclei. That is, 

 at weak ionization the vapor nuclei of dust-free w r et air become efficient 

 in the presence of ions. There are but two steps in the curve, the initial 

 one scarcely leaving the axis, the other at about n = 15,000 to 20,000. 



42. Moderate radiation. Let the radium tube be attached to the outer 

 surface of the fog chamber. The curve which is obtained begins appre- 

 ciably slightly above dp/p = o. 24 (v 1 /v = i . 21, adiabatic drop from 76 cm. 

 about 18.4 cm.), but it scarcely rises until above 0.25. From this point 

 it also sweeps upward but can not get much above 70,000 to 80,000 

 nuclei per cubic centimeter, which condition is reached at about 0.28. 

 To make this curve rise into the hundred-thousands, i. e., to make the 

 vapor nuclei of dust-free wet air efficient in the presence of the ions, 

 the exhaustion must be carried to about o. 50, much beyond the lateral 

 limits of the diagram ; but the fog is then intense and without coronas. 

 Again there are but two steps, one very near, the axis not appreciably 

 influenced by the greater ionization and the other above = 70,000. 

 Persistent nuclei are not produced, however long the exposure. 



43. Strong radiation. If an ordinary X-ray bulb (4-inch spark) is 

 placed at a distance of about 50 centimeters from the fog chamber, the 

 condensation produced begins appreciably somewhat below 0.24 (v 1 /v = 

 i . 21 ; adiabatic drop from 76 cm. about 18 cm.) ; but the graph scarcely 

 rises until nearly 0.25, when the upward sweep into the hundred-thou- 

 sands begins. Exposure of a few seconds produces fleeting nuclei only ; 

 exposure of one or more minutes produces persistent nuclei. In spite 

 of intense ionization, the first step near the axis has scarcely risen; the 

 other is indefinitely high beyond the reach of coronas. 



44. Other nucleations. I have ventured to place the data of J. J. 

 Thomson (Phil. Mag., vol. v, 1903, p. 349) at T in the same chart. 

 They must be interpreted, however, relatively to Wilson's points (nega- 



