TUBGIDITY 59 



pressure than a ^ gram-molecular solution of glucose. A 

 method must therefore be devised which will render it possi- 

 ble to compare readily the osmotic pressures of electrolytes 

 and non-electrolytes. This can be done by means of any 

 unit of pressure. The mercury column may be used, or 

 large pressures may be expressed in atmospheres. Recently 

 Errera' has suggested a special unit for measuring osmotic 

 pressure, which he proposes to call the tonie. It is to be 

 equal to the pressure of one dyne upon a surface of one 

 square centimeter. For larger measurements he suggests 

 the term myriotonie, equal to ten thousand tonies. It is 

 difficult to see how this new unit possesses any advantage 

 over the mercury imit for practical work. For plasmolytic 

 purposes it is much more convenient to reduce all measure- 

 ments to terms of a molecular solution of a non-electrolyte. 

 Thus comparison becomes easy and the absolute pressure per 

 unit surface can be readily found from the relation 



M= 22.3 atmospheres, or 1695 cm. Hg, 



where M is the osmotic pressure of a molecular solution of 

 a non-electrolyte. Of course, in making up solutions of an 

 electrolyte for use by this method it must be borne in mind 

 that the desideratum is not a molecular solution of the elec- 

 trolyte, but a solution whose osmotic pressure will just equal 

 that of a given solution of a non-electrolyte. Thus a solu- 

 tion of NaCl whose osmotic pressure is, say, -^ M, must 

 be considerably more dilute than a ^^tr molecular solution 

 of that salt. 



III. THE PEEMEABILITT OP THE PEOTOPLASMIO LAYERS 



The often repeated statement that the protoplasmic layer 

 is not permeable to solutes needs to be modified as follows: 

 To some substances it is probably absolutely impermeable 



1 L. Eeeeea, " Sur la myriotonie comme xmitS dans les mesures osmotiqnes," 

 Extr. des Bull, de I'acad. roy. de Belgique, Vol. Ill (1901), pp, 135-53. 



