16 METABOLISM 



342 g. must be used ; a G.M. of KC1 requires 74 g., a G.M. of NaCl requires 58 g., 

 per litre in each case. DE VRIES also discovered that a G.M. of invert sugar, 

 cane sugar, malic acid, tartaric acid, citric acid, &c. in short, a G.M. of every 

 non-metallic organic substance soluble in water had exactly the same osmotic 

 value, whence we may deduce the law that equimolecular solutions are at the same 

 time isosmotic. It is worthy of note that the plasmolytic method may also be 

 employed in chemistry for the determination of molecular weights, as DE VRIES 

 (1888 b) has shown in the case of raffinose. Various formulae have been put 

 forward by different chemists for this sugar, each giving a different molecular 

 weight : 



1. CttH,, O lt + 3 H 2 ; M.W. = 396 



2. C 18 H 32 16 + 5 H a O; M.W. = 594 



3. C 36 H 64 O 33 + ioH 2 O ; M.W. = 1188. 



If now the same material be treated with equivalent plasmolytic solutions 

 of raffinose and cane sugar, we find that a 3-42 per cent, solution of cane sugar 

 is isosmotic and equimolecular with a 5-957 per cent, solution of raffinose. 

 But a 3-42 per cent, solution of cane sugar is the equivalent of o-oi G.M. to 

 the litre, therefore a 5-957 per cent, solution of raffinose must have a similar 

 G.M., and its molecular weight must be 595-7, a number which strikingly 

 resembles the second of the formulae quoted above. The correctness of this 

 conclusion has been confirmed lately by other methods. 



What is true of organic non-metallic substances is, however, far from being 

 so of all compounds. We might expect a solution of 101 g. of potassium 

 nitrate in 1000 g. of water to give exactly the same osmotic effect as 342 g. of 

 cane sugar; in fact, it has almost the same effect as 1-5 G.M. of cane sugar, in 

 other words, one and a half times as much as one would expect. DE VRIES (1884) 

 has, moreover, shown that in the case of many substances one may obtain one 

 and a half, two, or two and a half times the value of an equimolecular sugar 

 solution. He assumed, to use round numbers, the value of a known sugar 

 solution to be 2, and found that other substances gave, more or less exactly, the 

 numbers 2, 3, 4, and 5. These numbers may be termed, following PFEFFER'S 

 nomenclature, isosmotic co-efficients, for they indicate how much greater the 

 osmotic value of the substance is as compared with sugar = 2, and a knowledge 

 of such co-efficients is of the greatest value in all plasmolytic research. 



If we further bear in mind that these isosmotic co-efficients apply only to 

 dilute solutions, the fact that a solution of, e. g., potassium nitrate has a value 

 one and a half times as great as what we would expect from its molecular weight 

 becomes at once clearly explicable. From a consideration of countless instances, 

 modern chemistry has shown that in dilute solutions molecules in part dis- 

 sociate into their constituent ' ions ', each free ion having the same osmotic 

 value as the entire molecule, i. e. it attracts water with the same force as the 

 molecule does. The degree of dissociation depends on the one hand on the 

 degree of dilution of the solution, and on the other on the nature of the dis- 

 solved substance. In the end all the molecules may become dissociated, and 

 these isosmotic co-efficients give, not, it is true, an exact, but still an ap- 

 proximate, indication of the amount of dissociation which has taken place ; 

 their significance is purely practical and they are useful on the whole if we 

 desire to calculate the osmotic value of a definite solution. 



We have spoken above of a force with which the dissolved substance 

 and the medium, water, attract each other. This conception was until re- 

 cently generally accepted ; but nowadays, owing to the advance in general 

 chemistry, it has come to be considered an old-fashioned. Osmotic pressures 

 developed in cells are now explained by reference to certain characteristics 

 exhibited by all bodies in solution. According to VAN'T HOFF'S theory, one 

 must ascribe to substances in solution the same characteristics as are ascribed 



