Page:Popular Science Monthly Volume 80.djvu/436

432 neutral molecule, are thrown instantly to the plates M and N, their speeds in the field here used being something like five or ten thousand centimeters per second; but, when the field is off, these ions remain in the air between the plates, and, sooner or later, as their number increases, one or more of them comes into contact with a drop and sticks to it. If we look now at the changes which occurred in the experiment recorded, we see that the first change in the time in the field, namely, that from 12.45 to 21.85, represented the advent upon the drop of 2 unit charges, since the 16-second time which is found later in the table was skipped in this catch. All the other changes in the table, save one, namely, that from 34.8 to 16.0, represented the advent of single charges, and this one represents again the advent of a double charge since the 21.9 second speed was here skipped. This indicates that there are probably no ions which have a very large number of units of excess of one kind of electricity upon them, but it gives us no information as to whether the act of ionization consists in the detachment of only one elementary electrical charge from a neutral molecule, or of two or three; for, so long as the changes are occurring when the field is off, it is impossible to distinguish between the capture of a single ion carrying two or three units of charge, and the successive capture of two or three ions each carrying the unit charge.

It was necessary, therefore, to catch the ions at the very instant of their formation, or better, to catch a molecule in the very act of splitting up into ions. Accordingly, the experiment was modified as follows. By suitably adjusting the PD between the plates M and N, it was found possible to hold a minute positively charged drop suspended, like Mohammed's coffin, as long as desired between heaven and earth, that is, in this case between M and N, the downward pull of gravity being exactly neutralized

by the upward pull of the field. Having obtained a drop in this position, there was produced beneath it a sheet of X-ray ionization in the manner shown in Fig. 3, so that when the X-ray bulb was excited, the drop was in a veritable shower of the charged positive residues of the molecules broken up by the X-rays. Now, if two or more electrons were