38 MOLECULAR WEIGHT AND POLYMER1SM 



has been used for actual measurements, viz, so far, mostly 

 membranes occurring in animal and vegetable tissues, and 

 the precipitate discovered by Traube. 



The membranes in animal and vegetable organisms, which 

 offer little resistance, have proved especially useful as 

 semipermeable partitions, and an example of each will be 

 given here. In the first place we have the protoplasm 

 sheath of the plant cell *, an elastic membrane resting 

 freely against the cell-wall, being kept against it by the 

 osmotic pressure of the contents. If, however, the cell, or 

 a layer of cells, suitable for microscopic observation be 

 placed in a salt solution of high osmotic pressure, the sheath 

 contracts away from the cell-wall : the process known as 

 plasmolysis occurs, as may be very well seen when the 

 protoplasmic contents are coloured (Tradescantia discolor). 

 Different solutions that exercise so high an osmotic pressure 

 as just to produce plasmolysis, are equal in their osmotic 

 action, are so-called isotonic, and therefore, according to 

 Avogadro's law as extended to solutions, contain equal 

 numbers of dissolved molecules in the same volume. Thus 

 e.g. a solution containing 5-96 per cent, of raffinose (a sub- 

 stance whose molecular weight was then unknown) was 

 found isotonic with a solution of cane-sugar (C^H^O^ = 

 342) containing 01 gm. mol. per litre (i.e. 3-42 / o ); hence 

 the molecular weight of raffinose must be approximately 596 



3.42:5.96 = 342:;*; x = &6, 



and as for raffinose the choice lay between C 12 H 22 O n 

 . 311.0 = 396 (Berthelot and Ritthausen), C 18 H 32 O 16 . 5H 2 O 

 = 594 (Loiseau and Scheibler), and C 36 H 64 O 32 . ioH 2 O = n88 

 (Tollens and Rischbiet), this was decisive in favour of the 

 second formula, a conclusion which has since been verified 

 by chemical means, since raffinose takes up water and 

 decomposes into three molecules of sugars, each containing 

 six carbon atoms (glucose, laevulose, and galactose) : 



1 De Vries, Zeitschr. f. Phys. Chem. 2. 440. 



