CHAPTER II. 



THE TRANSPIRATION OF THE SYSTEMS AIR-AIR, HYDROGEN-HYDRO- 

 GEN, AIR-HYDROGEN, HYDROGEN-AIR, ETC., THROUGH WATER. 



10. Introductory. Apparatus. — In the preceding chapter preliminary 

 data were given for the molecular transpiration of air, obtained from an 

 eleven-year period of observations of the increase of weight of a Cartesian 

 diver. This apparatus was ill-adapted for the experiments, because of its 

 small mouth. Consequently cylindrical swimmers have since been in- 

 stalled, both for air and for hydrogen, and often showed sufficiently rapid 

 progress to admit of a statement of results after several weeks. In fig. 3, 

 vd is the diver in the column of water A, usually resting in an elevated 

 position on the vertical wire-gauze partition, e. The imprisoned air is 

 shown at v in contact with the lower water-level, and / is the level of the 

 free surface of water. The 

 tubes a and b, the latter con- 

 taining a glass stopcock, are 

 useful in exhaustion, or in 

 special experiments for the 

 conveyance of an artificial 

 atmosphere of hydrogen into 

 the space above the free sur- 

 face/. T is the thermometer 

 placed eccentrically. The 

 heads //, h", ti" will be re- 

 ferred to below. 



This form of apparatus 

 is suitably modified in the 

 way shown in fig. 4, with 



Fig. 3. — Cylindrical 

 diver adjusted for dif- 

 fusion measurement. 



Fig. 4. — Cartesian diver 

 with double tube. 



a view to making uniform the section of the column of water through 

 which diffusion takes place. Here the swimmer vd is contained in a central 

 tube cd (with a stopcock at c) full of water. The swimmer fits the tube 

 with just sufficient freedom to slide easily. The tube is then partially 

 surrounded by the water in the larger vessel A . There may be a stop near 

 the top at/ to determine the level of flotation. This is particularly neces- 

 sary, both here and in fig. 3, when the top of the swimmer is flat. The 

 advantages of this form are many; in the first place, the section r and the 

 annular section r'of the diffusion column may be made the same throughout, 

 within, around, and above the swimmer, which is not the case in fig. 3; 

 the level / may be sharply determined, since there is no danger of the rider 

 parting the water at the surface. Discrepancies due to friction of convec- 

 tion currents are diminished. The heads h' , h" , and h'" may be more 



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