108 DISPERSE SYSTEMS 



to 3 per unit time. That is, surface area has been decreased and 

 surface tension increased. The final tube indicates graphically that 

 a balance may be struck between the mono- and the divalent 

 cations, producing a result differing not one whit from that shown 

 in the first tube where both chlorides were absent. In our later 

 studies we shall see in just how many reactions monovalent cations 

 like sodium, potassium and guanidine are antagonised by divalent 

 cations like calcium and magnesium. 



The behaviour of sodium and calcium soaps in emulsion-making 

 throws light on some peculiar problems in physiology. Loeb and 

 his co-workers found that certain marine organisms died when 

 put into fresh water. This will not appear surprising to the 

 student who remembers the phenomena of endosmosis, e.g., plasmo- 

 lysis, haemolysis, etc. That this explanation is not correct is 

 shown by putting the organisms into solutions of sodium chloride 

 or of calcium chloride having the same osmotic pressure as sea 

 water. If, however, the organisms which would have been 

 killed by immersion in these isotonic solutions were placed in a 

 solution having a definite ratio between the amount of sodium 

 and calcium present, life was maintained quite normally. All 

 protoplasm may be considered as an emulsion of lipoid material 

 in a colloidal-crystalloidal complex. The presence of the sodium 

 soap formed by interaction with the lipoids causes the formation 

 of a lipoid-in-water emulsion, while the calcium soaps emulsify 

 water-in-lipoid. The two types of emulsion thus formed are in 

 equilibrium with an environment containing a definite Na/Ca 

 ratio, that of sea water. Alteration in this ratio upsets the balance 

 between the two types of emulsion and causes the cessation of 

 growth and subsequently of life (see Nerve, Chap. XVIII.). 



Soap, above certain concentrations, exists as neutral undis- 

 sociated colloidal matter with a certain amount of water of 

 hydration " bound " in it. If the concentration of the soap is 

 decreased, some of it will become ionised, and so cause the " free " 

 water of solvation to give an alkaline reaction to litmus. Heating 

 a neat soap causes it to liquefy, i.e., to form liquid crystals. Mac- 

 lennan (1923) carried out work on the microscopic structure of 

 soaps, much of which is of interest to physiologists. He showed 

 that liquid neat soap had some kind of molecular structure or 

 orientation indicated by its power of rotating the plane of polarised 

 light (q.v.), i.e., the liquid is anisotropic like a solid crystal. The 

 crystal structure also produces characteristic X-ray photographs. 

 Soap solutions may be broken up in various ways. 

 (a) The addition of an acid stronger than the fatty acid frees 

 the fatty acids, e.g. H2SO4 + 2NaA = NagSO^ + 2HA. 



