OSMOTIC PRESSURE 159 



of active elements, or effective molar concentration, and its osmotic pressure will be 

 only 3 '4 mm. of mercury. 



In the case of substances which are colloidal on account of the large size of their single 

 molecules, as appears to be the case with haemoglobin, it is impossible to obtain solutions 

 of any great molar concentration. According to its content in iron, the molecular weight of 

 haemoglobin is 12,000 to 14,000, so that a O'Ol molar solution would contain 12 per cent, 

 of solid. Colloidal solutions of such strength cannot often be obtained, and a decimolar 

 solution would be solid. 



The above considerations appear to me to place a difficulty in the way of accepting 

 Roaf's view (1912, 1) of a cell membrane semi -permeable only as regards colloids. Plant 

 cells usually contain solutions with an osmotic pressure of 4 '5 atmospheres, which is that of 

 a 0'2 molar solution ; if a protein salt, even of so low a molecular weight as 2,000, is to afford 

 this pressure, a 40 per cent, solution would be necessary. This is higher than the total 

 solid content of protoplasm. More difficulties seem to be attached to this view than to that 

 of a true semi-permeable membrane, although it is suggested as a simpler one. If we are to 

 admit semi-permeability as regards glucose,. or non-electrolyte crystalloids, it is not a great 

 step to extend it to certain salts or even acids and alkalies. 



The first clear proof that colloidal solutions have a measurable osmotic pressure 

 was given by Starling (1896 and 1899) in the case of the colloids of blood serum. 

 A portion of serum was filtered through gelatine by pressure; this filtrate contained 

 all the crystalloid constituents of the serum, since gelatine holds back the colloids 

 only. The filtrate was placed in an osmometer with a gelatine membrane, while 

 on the other side of the membrane a portion of the unfiltered serum was situated. 

 Any difference in osmotic pressure observed must be due to difference of molar 

 concentration, and this again only to substances in the colloidal state. It was 

 actually found that the colloids in blood serum gave an osmotic pressure of 

 about 30-40 mm. of mercury. This fact will be found in later pages to have 

 an important connection with the secretion of urine. 



Moore and Parker (1902) measured the osmotic pressures of egg-white, serum, and soaps, 

 Moore and Roaf (1907) those of serum proteins, gelatine, and gum acacia. Hiifner and Gansser 

 (1907), and Roaf (1908), independently, made exact determinations of that of hfemoglobin. 



Some confusion has arisen as to the genuine nature of the osmotic pressure 

 obtained in the case of colloidal solutions on account of the difficulty of ensuring 

 the absence of electrolytes or other impurities of low molecular weight. It was 

 thought that, in some way, these foreign substances, although capable of free 

 diffusion through the membrane, might be held back by the colloid and thus 

 afford the osmotic pressure observed. Consideration will show that this cannot 

 be the case. If these foreign substances are in chemical combination with 

 the colloidal one, they are obviously part and parcel of the colloidal particles, 

 and not to be reckoned as impurities. Even if merely adsorbed, they are fixed 

 for the time on the surface of the colloidal particles, and are inseparable from the 

 colloidal elements to whose molar concentration the solution owes its pressure 

 they are, in fact, not free to exercise their own osmotic pressure ; while that due 

 to the colloidal substance will rather, if anything, probably be slightly decreased, 

 if the impurities are salts, owing to the increased aggregation of the colloidal 

 particles. If, again, these foreign substances are free in solution, they will diffuse 

 until equal in concentration on both sides of the membrane, and therefore inactive 

 osmotically. There is, however, one special case to which reference has already 

 been made (page 120 above), where both the colloid and the diffusible substance 

 are electrolytes. But here the concentration of the diffusible salt becomes less 

 in the presence of the colloid, so that it leads to a fall in the apparent osmotic 

 pressure on the part of the colloidal solution. Foreign diffusible substances 

 cannot, therefore, be held responsible for the actual experimental facts. 



Hiifner and Gansser (1907, p. 209), moreover, find that the osmotic pressure 

 of haemoglobin corresponds to its molecular weight, calculated on the basis of 

 its iron content. 



Moore and Roaf (1907, p. 63) noticed that the addition of sodium hydroxide 

 to a protein solution caused the osmotic pressure to rise, and interpreted the 

 fact as due to the formation of a salt with smaller " solution aggregates " than 

 the original protein. Now, in order to investigate the interesting and important 

 phenomena shown by electrolytically dissociated salts, of which one or the 



