ABSORPTION OF MATERIALS IN GENERAL 119 



Potassium sulphate has an isosmotic coefficient of 2 at the concentrations 

 employed by de Vries, the molecule of this electrolyte dissociates into three ions, 

 K, K and SO4, and the coefficient 2 indicates, in this case also, that half the total 

 number of molecules are to be considered as dissociated. The number of par- 

 ticles in solution would thus be about doubled, for ^ + 3 X Y2 = 2. 1 



DeVries used salt solutions of about 0.1-volume-molecular concentration, 

 these being about half dissociated. The degree of dissociation varies with the 

 concentration, and so the osmotic coefficients obtained by de Vries cannot be 

 used for solutions of other concentrations, the coefficients for which must be 

 obtained through the use of isosmotic solutions, 1 employing a solution of an 

 undissociated and unhydrated substance as a standard. 



Errera 2 proposed the myriotonie as a unit for the measurement of osmotic 

 pressure, to replace the arbitrary one of an atmosphere. A tonie is the pressure 

 exerted upon a surface of 1 sq. cm. by 1 dyne (the well-known unit representing 

 the force necessary to give a velocity-acceleration of 1 cm. per second to a mass 

 of 1 g.). The terms dekatonie, hectotonie, kilotonie and myriotonie (10,000 

 tonies) are employed for greater pressures. A myriotonie (M ) is about one 

 one-hundredth of an atmosphere." 1 



§5. Absorption of Dissolved Substances. — Only a few direct experiments 

 upon the entrance of dissolved substances into the cell are available. Some con- 

 clusions concerning the mechanism of absorption may be drawn from plasmolytic 

 experiments with salt solutions. Every substance entering the cell must pass 

 through two membranes, the cell wall and the protoplasmic membrane. Most 

 dissolved substances easily penetrate the cell wall, but the protoplasm is imper- 

 meable, or nearly so, to many of these. 



The osmotic properties of the protoplasmic membrane are similar to those 

 of Pfeffer's precipitation membranes. Only the living protoplasm is here 

 meant, however; dead protoplasmic membranes have entirely different proper- 

 ties. Thus pigments are persistently retained within the cell sap by the living 

 protoplast, but these and other dissolved substances diffuse out very rapidly 

 after the cell is dead. Like precipitation membranes, the protoplasmic mem- 

 brane is not completely impermeable to most substances. For example, Pfeffer 3 



1 Hamburger, H. J., Osmotischer Druck und Ionenlehre in den medicinischen Wissenschaften. 3 v. 

 Wiesbaden, 1902-1904. Hober, Rudolf, Physikalische Chemie der Zelle und der Gewebe. 2 Aufl. Leipzig, 

 1906. [4 Aufl. Leipzig, 1914.] Brasch, Richard, Die Anwendung der physikalischen Chemie auf die Phy- 

 siologie und Pathologie. Wiesbaden, 1901. 



2 Errera, L., Sur la myriotonie comme unite dans les mesures osmotiques. Recueil Inst. Bot. Bruxelles 

 5: 193-208. 1902. 



3 Pfeffer, W., Ueber Aufnahme von Anilinfarben in lebenden Zellen. Untersuch. Bot. Inst. Tubingen 

 2: 179-331. 1886-1888. 



1 The degrees of dissociation are actually much greater, however, than are assumed in 

 this discussion. DeVries's isosmotic coefficients are now to be regarded as of historical 

 interest only. The best discussion of the calculation of osmotic values of solutions is that 

 of Washburn, 1921. [See note e, p. 109.]. — Ed. 



m This unit has never come into general use and it is now highly improbable that it ever 

 will. Pressures are generally stated in terms of millimeters or centimeters of a mercury column 

 or in atmospheres, an atmosphere being 760 cm. of mercury. It seems undesirable to state 

 osmotic pressure in any other terms than those already used for other kinds of pressure. — Ed. 



