PREFACE. in 



trometer. For this purpose the whole condenser, as described in Chapter 

 III, is submerged in a clear non-conducting paraffin oil, while the mov- 

 able disk of the electrometer is floated on a Cartesian diver, or the circular 

 top of a cylindrical diver is itself the disk. The difference of weight of a 

 charged and uncharged condenser is determinable, the former in view of the 

 electrical pressures being less. It may then be shown that the absolute 

 difference of potential of the plates, c(Bt. par., varies as their distance apart 

 and as the square root of the difference of the manometer pressures which 

 are just compatible with flotation, in the case of the charged and uncharged 

 condensers, respectively. By keeping the difference in question constant, 

 potentials may be absolutely measured in terms of the distance apart of the 

 plates from about 50 volts to indefinitely large magnitudes. 



These experiments suggested a variety of other methods. Thus the disk 

 of the absolute electrometer, now kept in air, was buoyed up and held in 

 place on a hydrometer, with its body submerged in water or in oil, where the 

 capillary forces are small. Particularly interesting results were obtained 

 when the hydrometer was a very thin, straight aluminum tube, at right 

 angles to the light aluminum plate of the condenser, the aluminum tube 

 being submerged in a glass tube which is one shank of a U-tube. It is shown 

 that for a difference of potential of the disks (supposed horizontal) , not too 

 large, there is a stable and an unstable position of the movable disk, the 

 former below the latter. The disk therefore rises from its fiducial position in 

 the uncharged condenser to a definite height. As the difference of potential 

 increases this height increases until at a transitional height both stable and 

 unstable positions coincide. For greater differences of potential the disk 

 passes without intermission from the lower plate (guard ring) to the upper 

 plate of the condenser. If the difference of potential is constant, the same 

 phenomena may be evoked on diminishing the distance apart of the plates 

 of the condenser, by lowering the upper plate on a micrometer screw. 

 Potentials may then be absolutely measured in terms of the distance apart 

 of the plates at which the continuous rise of the disk first occurs. 



Other similar experiments were devised, such as the treatment of Cou- 

 lomb's law when one of the repelling bodies is a Cartesian diver, the repe- 

 tition of Mayer's experiments when the charged metallic bodies are floated 

 in oil in a charged guard ring, etc. 



Finally, the experience gained in Chapter III, in relation to methods of 

 filling the diver with a gas in an environment of the same gas, a condition 

 rigorously necessary if the gases are to remain adequately pure for diffusion 

 measurements, suggested the further development of certain of the experi- 

 ments in Chapter II. These are given in Chapter IV. In addition to this, 

 the chapter begins the work of treating the diffusion of gases through solu- 

 tions systematically and at length. It contains the effect produced on the 

 diffusion coefficient of air by dissolving in water different quantities of KCl, 

 NaCl, CaCl2, BaCla, SrClo, K0SO4, Na2vS04, FeCls, AICI3, etc. The purpose 

 here is at present chiefly the gathering of data. The work is so laborious, 



