TEE KINETIC THEORY OF MATTER 427 



stituent negative electricity — an electron — so that the residue of the 

 molecule is probably just like the neutral molecules of the surround- 

 ing 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 mole- 

 cule, 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 decompo- 

 sition 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 favof 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 atmos- 

 pheric ions which are responsible for all the manifestations of atmos- 

 pheric electricity which have been the object of man's awe and wor- 

 ship 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 experi- 

 ments, 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 G, 

 and one or more of the oil droplets was allowed to fall through a pin 

 hole at p into the space between 21 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 diiferent parts of the appa- 

 ratus in the experiment now imder 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- 



