the latter being secured in any position by the snugly fitting screw at C. The top of the spindle B is ground into the upper aperture of the body like an ordinary screw valve, so that steam may be quite shut off or supplied in any reasonable volume at any pressure by turning the head of the spindle. The bottom of the spindle receives the T-tube GEF, the joint being ground so as to admit of rotation of the spindle around the tube. Air charged with nuclei

is conveyed through this tube, entering at F and coming from the identical gasometer train and phosphorus tube already described.

To obviate the danger of steam entering the tube, EF, and quenching the ionizer, a hollow nozzle, D, is ground into the top of the spindle and removable at pleasure. This introduces the dust at 1 cm. above the annular opening in the jet, where the pressure excess is smaller. The nozzle does not otherwise interfere with the action. Should water enter the tube E, it may be removed by opening the stopper G.

The color tube as now modified is shown at C, figure 8, the jet playing

directly into the open end of the tube, which is telescoped (not shown) so that the lower section may be raised to facilitate access to the jet. The mirror for illumination is at m, n being the observation window cleared through a, and e the escape steam pipe.

The bottom of the spindle is joined by a thin tube, t, called absorption tube, about 1 millimeter in diameter to the phosphorus ionizer, P, which here consists of a tube about 30 cm. long and 1 cm. in diameter, charged in the manner described above. Fis the screw stop-cock, D the desiccator, U the pressure gauge, the volume flask, M the large Mariotte flask.

Figure 9 is a slender tubular condenser by which the absorption tube t may be replaced for correlative electrical measurements. See Chapter 5.

11. Results. In table 7, I have given several series of results obtained at different times. Different absorption tubes at t were used, and hence the results can only be compared by putting the volume of charged air producing the standard blue equal to one unit. Variations between the series necessarily remain, because it is impossible to select the same shade of blue from a continuous series. The pressure of the steam jet is given under p; is the air temperature, and P the pressure excess in the volume flask in cm. of mercury, Reduction to standard pressure and temperature is superfluous.

TABLE 7.-COLOR AND RELATIVE NUMBER OF PARTICLES IN CASE OF NEW JET (AXIAL INFLUX). "BLUE"= 1.

as

These observations are constructed in the chart figure 10, by representing abscissas the thicknesses of air-plates which would give the same color by Newton's interferences for normal incidence and transmitted light, as no better method is immediately available. The ordinates are the volumes per minute of charged air producing the same color in the tube. The curves are taken directly from the chart, figure 3.

12. Discussion.-To turn first to table 7, it is seen that whereas in the earlier work the dust contents needed to produce a blue field was but .5 liter per minute on the average, the requirements here are as high as 2 lit./min. and not below 1.2 lit./min. On the average even three times as much dust is needed. This is a disadvantage of the present form of jet, inasmuch as the law of absorption in the tube t, Fig. 9, has now a fundamental bearing on the data obtained from the apparatus, which will in many cases outweigh the convenience of stronger colors of the second order. The jet, moreover, is not available for the capture of atmospheric nuclei. Different purposes will therefore be subserved by the two forms of jet.

If these series of observations are examined individually as far as blue, they will be found to lie on a line with somewhat less concavity upward than was the

FIG. 10.-CHART OBTAINED WITH NEW JET CORRESPONDING TO FIGURES 3 AND 4.

case above. Their regularity, even for color criteria, is not as great as would be anticipated, particularly in the region of the second order (right end of curve). As the observations were taken on different days, it seems to me that the state of the atmosphere must impress itself upon these measurements, as the effect of atmospheric nuclei would be largest on the right where the supply of artificial nuclei is smallest.

Compared with the preceding results (figure 3), the present series of data lie quite within the same margins of values. Indeed, the results with the jet dusted in the great variety of ways from without, the nuclei being borne into the jet on a

convection current often traversing many feet to meet it, and the results here, when the active dust is at once introduced into the jet from within, are indistinguishable on the whole, provided the same color value (blue=1) be given to one of the colors. The absolute volumes of charged air (liters/min.) evoking the color may in different experiments vary over threefold. The reason for this, the absorption of the tube t, figure 9, will appear in the next chapter.

CHAPTER III.

TRANSMISSION OF THE IONIZED EMANATION OF PHOSPHORUS THROUGH AIR IN THE ABSENCE OF AN ELECTRIC FIELD.

1. Introductory.-For reasons of both theoretical and practical import, it is next necessary to ascertain the precise conditions under which the phosphorus nucleus vanishes on passing through tubes at a definite velocity; or in general on

1

FIG. 1.-COLOR TUBE AND ABSORPTION TUBE. SCALE

being retained in any vessel or put in contact with any barrier in a definite way, for a definite time. The experiments of the present paper thus relate to the absorption of condensation producing atmospheric nuclei by surfaces or by suspended particles. They show, I think, that such absorption takes place as though each nucleus of a nearly saturated region travelled in the entire absence of an electric field, with a velocity of about 3 millims. per second; or if it be put roughly that of the total number travel in a given cardinal direction, as though each nucleus had a velocity of about a centim. per second.

'Cf. Phil. Magazine, (6), vol. II., p. 40, 1901; Am. Journ. Sci., (4), xi., p. 237, 1901; Science, xi., p. 1, 1900.