158 SCIENCE AND FRUIT GROWING 



25 per cent. ; these limits, however, varying with the nature of 

 the soap. 



A more satisfactory explanation of emulsification was found 

 in the view that it is due to the presence of minute, solid particles 

 in the liquid, such particles being, according to well-known 

 laws, attracted by the oil globules, and forming a coating or 

 pellicle over them, thus preventing them from coming into 

 contact with each other and coalescing unless, however, the 

 particles are wetted more easily by the oil than by the water, 

 for in that case they will enter the globules instead of remaining 

 on the surface of them : this is so with sulphur, purple of Cassius, 

 etc., which, though very finely divided, will not emulsify paraffin. 

 It was found that the mere addition of a very finely divided 

 insoluble substance to water rendered ihe emulsification of 

 paraffin possible, and amongst such substances the basic sul- 

 phates of copper, iron, and other metals were conspicuous. 

 When any such emulsifier was deeply coloured, it was possible 

 to see under the microscope the dark coating of particles en- 

 closing the globules. Just as this coating prevents the globules 

 from coming into contact with each other, so it prevents them 

 from coming into contact with, and " wetting," the vessel 

 containing them : it is one of the most certain tests that 

 emulsification has occurred to find that, though containing so 

 much oil, the liquid does not make the vessel oily, any more 

 than milk does. 



To render such an explanation acceptable, there must be 

 solid particles present in a soap solution, since it produces 

 emulsification ; and such particles certainly are present, for 

 soap in water always decomposes partially, becoming cloudy 

 through the separation in minute particles of an " acid " soap. 

 Such a separation, it may be mentioned, occurs less readily in 

 strong solutions, and this explains why a soap solution, when 

 above a certain strength, will not emulsify oil. 



(3) De-emulsification. Emulsions with solid emulsifier s. 



Anything which will destroy the coating of solid on the oil 

 globules must result in de-emulsification ; a trace of acid will 

 do so in most cases, though not if the solid is insoluble in acid, 

 as, for instance, where finely divided clay is the emulsifier. 

 With a soap solution the solid present is not in a very stable 

 condition, but is continually redissolving, and then separating 

 again from the liquid, the result of which is that the globules 

 occasionally find an opportunity of coalescing, and hence in time 



