162 PRINCIPLES OF GENERAL PHYSIOLOGY 



itself was noticed by Graham (1861, p. 217) in connection with the sodium salt of " albumen," 

 where it was found that all the sodium diffused away in process of time and was found in the 

 outer water in combination with carbon dioxide derived from the air. The same thing 

 happens with the salts of caseinogen. It is necessary to give this warning, since various 

 incorrect statements have been made on the basis of experiments in which this factor was 

 ignored. It is, for example, no proof of hydrolytic dissociation when sodium is found to have 

 diffused out. 



The osmometer of Moore and Roaf (1907), with the additions described by myself 

 (1909, i. p. 271), will be found suitable for the investigation of colloidal solutions. The 

 platinum lining is rarely necessary ; it will be found sufficient to have the inside electro- 

 gilt. The membrane ma}' be of parchment paper, or of this impregnated with gelatine, 

 collodion, etc. 



Many proteins, as we have seen (page 104 above), take up water by imbibition. 

 In theory it would seem, therefore, when a certain molar solution is made, that 

 the solution is really more concentrated than was intended, owing to the taking 

 up of water by the colloid, which water is then no longer free as solvent. The 

 osmotic pressure would, for this reason, be higher than the theoretical one. It 

 is difficult to state how far this is actually the case, since we are so much in the 

 dark as to the true molecular weight of proteins. The case of haemoglobin, which 

 has an osmotic pressure in agreement with its molecular weight, suggests that the 

 effect of imbibition is negligible. Some measurements of the osmotic pressure of 

 the sodium salt of caseinogen made by myself (1911, i. p. 234) agree with the molec- 

 ular weight assigned by Laqueur and Sackur (1903, p. 199). It may be that, 

 although each molecule of the protein takes up a considerable number of water 

 molecules, the total number of protein molecules present is too small to affect 

 appreciably the molar fraction of the water, which is always present in excess. 

 Pauli (1910, p. 485)j however, is of the opinion that the process of imbibition plays 

 an important part in the apparent osmotic pressure of proteins. 



Since the manifestation of osmotic pressure is an aspect of the kinetic energy of particles 

 in motion, which also shows itself in the power of diffusion through a liquid, it is interesting 

 to note that ovedberg (referred to by Arrhenius, 1912, p. 27) found that a certain gold 

 hydrosol had a diffusion constant of 0'27 per day ; in the same units, chlorine, bromine, and 

 iodine have respectively values of 1'22, 0'8, and 0'5. There is, then, more difference between 

 the rates of chlorine and iodine than between those of iodine and of gold particles. 



RELATION TO CELL PROCESSES 



Living cells, as we saw in the previous chapter, are surrounded by a semi- 

 permeable membrane, so that it is obvious that the osmotic pressure of the 

 solution outside, compared with their own osmotic pressure, is of great importance 

 in many ways. 



The osmotic pressure in the interior of such an organism as an Amoeba must be higher than 

 that of the fresh water in which it lives. Hence, if the cell is covered by a semi-permeable 

 membrane, water is continually being taken up into its substance. According to Stempell 

 (Zool. Jahrb. Abt. Zool., xxxiv. (1914) pp. 437-478), it is the function of the contractile 

 vacuoles of these organisms to get rid of, periodically, the water which has entered in this way. 



There is, we may note in the next place, an important difference between 

 vegetable and animal cells. The former, surrounded by a tough cellulose envelope, 

 are usually surrounded by water or by a considerably hypotonic solution ; in this 

 way their internal osmotic pressure is uncompensated and maintains a state of 

 tension or " lurgor " in the cell, serving to keep up the more or less rigid condition 

 of living plant structures necessary for their satisfactory exposure to air and light. 



Animal cells, on the contrary, are, as a rule, free to change their dimensions 

 by taking or giving up water. In order that they may remain in a normal state, 

 therefore, they must be surrounded by an isotonic solution. Now, any substance 

 in appropriate concentration will make an isotonic solution, provided that the 

 cell membrane is impermeable to it. On the other hand, there are very few 

 substances which have no action on the cell beyond that due to their osmotic 

 pressure. Perhaps cane-sugar has the least action, but, as we saw above (page 125), 

 it is not a completely indifferent substance. The effects of solutions which are 

 merely due to their osmotic pressure are accordingly rather difficult to investigate. 

 In certain cases, however, the state of affairs is quite clear. 



