stituent negative electricity—an electron—so that the residue of the molecule is probably just like the neutral molecules of the surrounding gas, save that it now carries a free or unbalanced positive charge corresponding to the negative charge of the electron which it has lost. The escaped electron probably soon attaches itself to a neutral molecule, so that shortly after the decomposition of a molecule, the gas is in the same condition as it was before the decomposition, save that two of its previously neutral molecules are now electrically charged, one positively and the other negatively. Whether this molecular decomposition which goes on continually in ordinary air is due to rays from traces of radio-active substances, which are present at all times in the air, or whether it is due to an occasional spontaneous explosion of a molecule, we can not as yet be absolutely certain, though the evidence is at present strongly in favor of the former hypothesis. But, however they may be formed, there can be no doubt of the presence of these ions in the atmosphere at all times, to the extent of from 1 to 15 per cubic millimeter, nor can there be any doubt that it is these atmospheric ions which are responsible for all the manifestations of atmospheric electricity which have been the object of man's awe and worship throughout all ages.
Now the problem which was set for this investigation was to catch individual ones of these atmospheric ions and to find what sort of charges they possess. A detective which could be set on the trail of a thing so small had evidently to be a distinctly undersized member of the force. It was in fact an oil-drop so minute as to be little more than visible through the most powerful microscope. In these experiments, however, no such high-power microscope was needed, for in a sufficiently powerful beam of light the oil droplet could be made to appear as a bright dot even to the naked eye in spite of its minuteness. The method of setting it at work was this. A spray of oil was blown from an ordinary commercial atomizer A into a dust-free chamber C, and one or more of the oil droplets was allowed to fall through a pin hole at p into the space between M and N. As it floated there, slowly falling under gravity, it was illuminated by a powerful beam from an arc light, which passed through diametrically opposite windows in the encircling ebonite strip c. It was viewed through a third window placed on the emergent side of the beam about fifteen degrees from its direction. A glance at the accompanying photograph, which shows a modification of the device, used for work at low pressures (see below), will make clear the arrangement of the different parts of the apparatus in the experiment now under consideration. The appearance of this drop of oil in the observer's short focus telescope through which it was viewed was that of a brilliant star on a black background. Be-
