204 THE BODY AT WORK 



directly proportional to the degree of concentration of the 

 solution. If the solution contain 1 per cent, of sugar, a 

 pressure of 500 millimetres of mercury is needed ; if it contain 

 2 per cent., a pressure of 1,000 millimetres ; if 6 per cent., 

 of 3,000 millimetres. 



In the next experiment separate two solutions, A and B, by a 

 hemipermeable membrane. Let A contain one salt only X ; 

 let B contain several salts X, Y, Z. Water will pass from A 

 to B, or vice versa, unless the osmotic pressure of the salts which 

 the solutions contain is the same,. The osmotic pressure will be 

 found to be the same if the total number of molecules dissolved 

 in A equals the total number of molecules dissolved in B. If 

 in A there be N molecules of X (per unit volume), and if in B 

 there be nX, n'Y, n"Z, the osmotic pressure will be the same 

 provided n + n' + n" = N. This, it will be seen, is a very different 

 matter from equality of percentage composition. Some mole- 

 cules are light ; others are heavy. The percentage weight of 

 X + Y + Z in B may be very different from the percentage 

 weight of X in A. To estimate the osmotic pressure of a 

 mixed solution, it is not sufficient to add together the per- 

 centages of the various salts which it contains. " Concentra- 

 tion," in the sense in which it was used in regard to blood and 

 urine, refers to the number of molecules of dissolved substances 

 in a given volume, not to their weight. 



It would be undesirable to attempt in this place to enter 

 upon the theory of osmosis. Enough has been said to suggest 

 to the reader that he should, when endeavouring to apply its 

 laws to the explanation of physiological phenomena, bear the 

 following facts in mind : Some membranes are permeable to 

 water and to the crystalloids which it dissolves ; others, 

 although permeable to water, are impermeable to substances 

 in solution. Some substances are diffusible through per- 

 meable membranes ; others are not. Osmosis of water occurs 

 from the solution of lower to the solution of higher concentra- 

 tion. Diffusion of crystalloids is their escape, owing to their 

 own molecular movements, from a situation in which they 

 are denser to a situation in which they are less dense. It must 

 be added, however, that various circumstances prevent the re- 

 duction of the laws of osmosis to simple terms the tendency of 

 salts to dissociate when in solution, their bases and acids acting 



