EMULSIONS 83 



soap is " salted out " as a curdy mass. The salt reduces the 

 hydrophilic powers of the soap and so reduces the stability of 

 the dispersoid. This is a different phenomenon from the precipi- 

 tation of a colloid by electrolytes. 



(c) On adding a soluble salt of calcium or magnesium to a soap 

 of ammonium, sodium or potassium a curdy precipitate is produced. 

 This curd is a calcium or magnesium soap, which, as we have seen, 

 has little or no affinity for water. 



(d) Solvents of soaps added to a water-soap dispersoid lead to 

 a partition of the soap between solvent and dispersion medium. 

 The effect of the anaesthetics on soap sols is interesting. Alcohol 

 brings about a rapid separation of soap and water, practically all 

 the soap dissolving in the alcohol. Chloroform has much the 

 same effect, but the partition is not so complete. To get anything 

 like a complete extraction large amounts of chloroform must be 

 used. Ether has hardly any effect. 



Soaps have a powerful effect in lowering surface tension, which 

 effect is greatly increased by the addition of small quantities of 

 alkali (Shorter and Ellingworth). A stalagmometer reading 

 of oil dropping into water was 65 drops. When 1 per cent, soap 

 was added to the water the drops increased to 260 (Hatschek). 



Emulsions. 



Although, as far as is at present known, emulsions do not take 

 a direct part in the energy exchanges of the body, and should 

 therefore not be treated in this section ; yet as they are so closely 

 allied to the colloids, and are really never found unless mixed 

 with colloidal or semi-colloidal matter, it is convenient to deal 

 with them now. 



An emulsion may be regarded as an emulsoid with somewhat 

 larger dispersed particles (microns). The term, as usually 

 employed, has, however, a narrower connotation, the disperse 

 phase being considered as a fat or fat-like substance distributed 

 throughout water in such a way as to remain stable for an in- 

 definite period. Oil and water are two immiscible liquids and 

 no amount of mechanical mixing will induce them to form a 

 permanent emulsion. It is true that after a prolonged beating 

 of the two together a maximum of 2 per cent, of the oil may be 

 taken up by the water, forming a stable dispersoid. Measurement 

 of the particles, however, demonstrates that they are of the order 

 of sub-microns, and thus a true colloidal system has been formed. 

 An example of this is the condensed water of steam engines, which 

 contains lubricating oil in suspension. 



