Butler and Sheridan — Account of a Neiv Oedanometer . 463 



We also include descriptions of two simpler forms of Oedanometers more 

 suitable for class demonstration. 



Hales recorded the fact that peas swelling in water could lift a weight of 

 83"5 kilos. Since the time of Hales a number of such records are to be found 

 scattered through botanical literature. Further, a number of papers, princi- 

 pally of a physico-chemical nature, discuss the theoretical aspects of swelling 

 organic bodies : for example, those of Rieeke (14), Schwendener (17), 

 Rodewald (15) ; and in some cases estimate theoretically, from thermodynamic 

 considerations, the pressures under which water enters swelling bodies — for 

 example, starch. 



Of the experimental determination of the pressures Reinke (13) gives the 

 most complete account, using an instrument which he terms an " Odometer," 

 which consists of a massive metal cylinder, bored perpendicularly for several 

 centimeters. lu this boring a piston travels. Tlie piston is pierced by 

 numerous fine holes serving to admit water. The upper end of the pistou 

 is continued as a strong rod, terminating in a platform, on which various 

 weights can be placed. The movements of the table-top are registered by a 

 lever travelling over a divided scale. The object to be tested is placed in the 

 hollow cylinder under the piston. A heavy weight is placed on the table, 

 and water is then admitted. The rise of the piston is registered by the 

 pointer on the scale, a portion of the load is removed, and when equilibrium 

 is again established, a second reading is taken, and so on. In this way a curve 

 is plotted, showing the increase of volume under a diminishing load. 



This apparatus is very satisfactory in dealing with bodies of a cylindrical 

 shape, which can completely fill the hollow cylinder ; it is not so suitable 

 for registering the pressure of irregular bodies like seeds. As will be shown 

 later, it is essential to fill the pore spaces with some incompressible, or slightly 

 compressible, yet porous body such as sand. If one looks at the mass of 

 seeds under the piston in figure 1, it will be seen that a rise of the piston 



TJ 



through a distance H will allow a total expansion of -= , where L is the 



length of the cylindrical mass of peas. 



Now the seeds, as will be shown later, only occupy | of this volume, so 



O TT 



that their increase in volume is - x — . Further, the layers of seeds near 



the piston will be freer to swell than the otiiers, owing to the friction of the 

 compacted mass of seeds. On both these accounts such an apparatus is not 

 likely to give very consistent results in the case of seeds. 



Grehant (8) used an apparatus consisting essentially of a strong 

 cylindrical vessel filled with seeds, and closed by a strong lid bolted or 



4:D2 



