CHAPTER I. 



THE TRANSPIRATION OF AIR THROUGH A PARTITION OF WATER. 



1. Molecular Transpiration of a Gas. — Ever since 1895 I have observed 

 that the Cartesian diver used in my lectures grew regularly heavier from 

 year to year. The possibility of such an occurrence is at hand; for the 

 imprisoned air is under a slight pressure-excess as compared with the 

 external atmospheric air. But this pressure gradient is apparently so insig- 

 nificant as compared with the long column of water through which the 

 flow must take place that opportunities of obtaining quantitative evidence 

 in favor of such transpiration seem remote. If, however, this evidence is 

 here actually forthcoming, then the experiment is of unusual interest, as it 

 will probably indicate the nature of the passage of a gas molecularly through 

 the intermolecular pores of a liquid. It should be possible, for instance, to 

 obtain comparisons between the dimensions of the molecules transferred 

 and the channels of transfer involved. 



2. Apparatus. — Hence on February 27, 1900, I made a series of definite 

 experiments* sufficiently sensitive so that in the lapse of years one might 

 expect to obtain an issue. The swimmer was a small, light, balloon-shaped 

 glass vessel, vd, fig. 1, unfortunately with a very 



narrow mouth 2 mm. in diameter at d, in the long 

 column of water A . The small opening, however, 

 gave assurance that the air would not be acci- 

 dentally spilled in the intervening years. For this 

 reason it was temporarily retained, the purpose 

 being that of getting a safe estimate of the con- 

 ditions under which flow takes place. 



In fig. 1, ab is a rubber hose filled with water, 

 terminating in the receiver R. Here the lower 

 level of water may be read off. Moreover, R is 

 provided with an open hose C, through which pres- 

 sure or suction may be applied by the mouth, for 

 the purpose of raising or lowering the swimmer, vd, in the column A. 

 this way constancy of temperature is secured throughout the column 



Fig. t. — Cartesian diver ad- 

 justed for diffusion meas- 

 urement. 



In 



3. Barometer. — The apparatus is obviously useful for ordinary baro- 

 metric purposes, and provided the temperature, /, of the air at v is known 

 to 0.025 C, the barometric height should be determinable as far as 0.1 mm. 

 Apart from this the sensitiveness of the apparatus is surprising. Great care 

 must be taken to avoid adiabatic changes of temperature, so that slow 

 manipulation is essential. These and other precautions were pointed out 

 in the original paper. The apparatus labors under one fundamental diffi- 

 culty, as the diffusion of a compound gas like air is a complicated discrep- 



"Am. Journ. Sci., ix, 1900, pp. 397-400. 



