288 PROPERTIES CONFERRED BY COLLOIDAL CONSTITUENTS 



The value of the critical ratio varies with different samples of oil, 

 because of their varying fatty-acid content. It also varies with the 

 proportion of alkali employed, since if the quantity of soap be insuffi- 

 cient to surround all the droplets with a layer of molecular thickness 

 the stable emulsification of the whole of a large excess of oil becomes an 

 impossibility, and the critical ratio is increased. 



It will be observed that upon passing the critical ratio the char- 

 acteristics of the emulsion change very markedly. Instead of the 

 yellow, fluid emulsions obtained while water is the internal phase, 

 creamy and more viscous emulsions result when water is the external 

 phase. In the neighborhood of the critical ratio the viscosity of the 

 water (oil) emulsions is very greatly enhanced, and emulsions of an 

 almost butter-like consistency may be obtained. This probably arises 

 from the fact that when the water in the emulsion is just, and only just 

 sufficient to surround all of the oil-droplets, any deformation whatever 

 of the tightly-packed oil-droplets must increase the size of the interstices 

 between them; but this can only be accomplished by a complete disrup- 

 tion and inversion of the emulsion, since the water is already stretched 

 to the utmost limit of its covering-power. The viscosity, or resist- 

 ance to deformation of these emulsions, therefore, represents the force 

 required to invert their structure. 



Not only the lipoid constituents of cells, but also the Proteins tend 

 to form films at the surfaces of suspended droplets, and thus facilitate 

 the formation of emulsions. If Chloroform be shaken up with pure 

 distilled water no emulsion arises; the two liquids separating com- 

 pletely after a very brief interval. If, however, a protein be added to 

 the water the chloroform, instead of separating out in the form of a 

 continuous layer, separates out in small discrete droplets which, if 

 numerous, form a milky layer at the bottom of the vessel; by trans- 

 mitted light, however, they appear perfectly transparent. These 

 droplets are extraordinarily stable and do not coalesce, however long 

 they may stand in contact with one another. They may be repeatedly 

 washed in water until all traces of protein have disappeared from the 

 wash-fluid, and they still remain perfectly stable and distinct from one 

 another. They may be shaken up in chloroform itself or treated 

 with dilute sodium hydroxide solution without impairing their form or 

 stability. If, however, they be heated to nearly the boiling-point of 

 chloroform under a layer of water the droplets burst and coalesce, 

 forming a homogeneous layer of chloroform. If treated with alcohol 

 they immediately dissolve leaving a fine membranous precipitate of 

 protein floating in the water. Thus if we shake up chloroform with 

 about twice its volume of a one per cent, solution of Protamine Sulphate 

 or a one per cent, solution of Gelatin, and, after allowing the droplets 

 to settle, pour off the supernatant fluid and repeatedly wash the drop- 

 lets with water, then if we suspend these droplets in a small amount of 

 water and add to the water an equal volume of Alcohol and gently 

 shake the test-tube, the droplets which are thus stirred up into the 



