PHYSICS. 273 



gained weight at a decreasing rate during the ensuing ten days, and there- 

 after assumed the steady rate of loss for months. Changes were as a rule 

 abrupt. It was found that a similar doubly inflected progression of results 

 usually occurs unless all manipulations at the outset are conducted, not in 

 air, but in a medium of hydrogen, or in general of the gas within the diver. 

 Otherwise the imprisoned gas is at once contaminated by diffusion of the 

 surrounding gas into it. It is not perhaps fully appreciated by chemists that 

 gases, otherwise pure, if stored over water, are at once contaminated with 

 air, by diffusion. In fact, a gas. A, in the swimmer, in presence of gases, C, 

 D, etc., can not escape by diffusion, until the sum of the partial pressures of 

 B, C, etc., is equal to or greater than the pressure equivalent of the head of 

 water under which the gas A is submerged. Before that the gas of the en- 

 vironment will diffuse into the diver against the hydrostatic pressure of the 

 head of water, i. e., apparently up hill. The same explanation accounts for 

 the enormous inflation of the microscopic air-bubbles in the liquid, when the 

 surrounding atmosphere is some other gas, like hydrogen. 



Other diffusion experiments, air into hydrogen, oxygen into hydrogen, 

 hydrogen into air, etc., were eventually pursued through months and com- 

 pleted in a similar manner and with similar results. The graphs obtained 

 are throughout striking. It is feasible to derive the differential equation for 

 these phenomena, but as might be expected, from the complications in ques- 

 tion, it could not be integrated. Finally it is interesting to note that if the 

 diffusion coefficients are given, the densities of the gases diffusing at a con- 

 stant rate may be computed ; or from another point of view, the degree of 

 purity of the gas so diffusing may be ascertained. 



The sensitiveness of weighing in case of the Cartesian diver, where the 

 whole apparatus is quite submerged in water or some other liquid and capil- 

 lary forces are out of the question, naturally suggested the application of 

 this method for the measurement of high potentials in case of the absolute 

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

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

 the movable disk of the electrometer is floated on a Cartesian diver. 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 dift'erence of potential of the plates, other things 

 being equal, varies as their distance apart and as the square root of the dif- 

 ference of the manometer pressures which are just compatible with flotation, 

 in the case of the charged and uncharged condenser, respectively. By keep- 

 ing the difference in question constant, potentials may be absolutely meas- 

 ured in terms of the distance apart of the plates from about 50 volts to in- 

 definitely 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 oil, where the capillary 



